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Skyway systems are predominately found in cities with cold climates, like the Minneapolis Skyway System:
But why would a city with a warm climate have a skyway system when it is not necessary for comfortable transport?
Edit: Assume that the world is Earthlike and breathable. People can freely walk outside without being uncomfortable.
Reasons:
Choosing an answer from the real world: because you have a lot of pedestrians, and without the elevated walkways they would effectively block all motor traffic.
This is the main reason for Tokyo's many elevated walkways. And Tokyo, despite having a "warm" climate and little snow, has a lot of them -- between buildings, paralleling streets, providing bridges over street crossings.
The walkways are also built for aesthetic reasons, either architecture or as parks, but the primary reason for them is to allow copious pedestrian traffic (Shibuya Crossing alone has 3000 pedestrians/minute) to not conflict with motor traffic.
So, if you just make your city sufficiently dense and pedestrian/transit oriented, you don't need a lot of additional justification.
In the UK many university buildings are joined by this sort of walkway. The reason is that, for a while, joining two or more buildings this way allowed them to be counted as a single building for tax purposes.
Many reasons, but the most important one is :
It's a very subjective reason, yet, it's true enough. Not all architecture is about being absolutely practical, even though contrary to pure art they have to be to some extent. For instance, why would you need a so high roof in the entrance of many ever-so stylised buildings? There are no furniture nor giant people who will need such height, yet people tend to like them for the sake of not being cramped up in a small place.
On a broader scale, an architect will want (or not) to build such a bridge for the same reasons. For instance, for a Megacorp building with its key letter "H", one could want to play with the bridge to form the letter. Even broader, bridges such as these could be seen as the latest trend in urbanisation, and to keep up with it they decide to build many of them.
Know that in urbanisation, a lot of factors are taken into account. For a concrete example, in my city, there was a choice which was given to the town council : An upcoming subway was going to go right through the middle of the city, and they had to choose whether it should be built under or above the ground. While the project was more cumbersome and costly if built underground, it was built below anyway because it was much, much more pleasing to the eyes and didn't cut the city in half. You can see these underground and above stations photos to give an idea of how big the difference was.
My point here is that a lot of architecture choices are also about visual appeal. And if roofed pedestrian bridges are pretty at some place, then I don't see why an artistic architect wouldn't suggest it in the district's rehabilitation plan! At worst, air refreshers such as fans or air conditionners will be needed, which will only add a minor cost to the project.
Off the top of my head, I can think of 2 reasons:
Additional 3rd point: it's your story and you want them.
Many of the buildings of your city are so tall, it is considerably time consuming to travel down to ground level, then back up to the appropriate level in the new building. So relatively high-level skyways link many of the buildings, and of course skyway floors are where shops, bars etc congregate.
You could even add some kind of class stratification to the different "tiers" of pedestrian links etc, or note how certain higher levels were "islands" that were too far from other clusters of tall buildings to link, so you had to go back to a lower level for the connection if it suits your story.
Mosquitos, perhaps carrying something unpleasant. Skyways allow people to get around without getting bitten.
Airco combined with shorter travel
Assuming a hot city maybe people want to walk in nice air-conditioned skyways. Also it would probably be better from an energy conservation view than constantly having to open the door and let the heat in.
Besides, it might just be shorter and reduces traffic congestion. As your picture nicely shows you can easily walk above traffic straight to where you want instead of having to go down extra stairs and wait for traffic to go to the other building.
It may be unsafe, unpractical or unpleasant for pedestrians to travel outside on foot. Some possible reasons:
Skywalks are for the elite. You need to pay a large sum of money to gain access to the Skywalks, with its luxury shops, fine dining restaurants and exclusive night clubs, which makes the inhabitants of the city covet access passes. If you have to walk outside, you certainly can't be a member of the country club!
The green-skinned people praying to Boreas use the skywalks, while the blue-skinned people praying to Eurus must keep to the outside streets.
Most cities tend to see a substantial change in ground level over time.
Usually, this tends to raise ground level. For example, when we repave roads (for example), we just layer the new pavement on top.1
But it can run the opposite way too: when we extract wellwater to drink, the ground (without water underneath to support it) subsides. Jakarta is subsiding so fast that Indonesia has changed the capital to another city;2 Bangkok is subsiding slower, but still noticeably. If your buildings are all anchored in bedrock that is not susceptible to the same subsidence trends, what was the first floor may become elevated.
Ground level shifts don't have to be a "natural" process either. Mid-20th century urban planners often built road trenches to avoid slowing their streets down with pedestrian crossings if they didn't have the money for a full tunnel. Before them, railways could build similar trenches (although they usually sprung for tunnels, since the ventilation cost differential was smaller). Maybe your city originally trenched its main thoroughfares…and then kept on trenching more and more streets. Eventually, the sidewalks outside the trench became a skywalk.
A real world example: A former employer of mine was headquartered in a warm locale that rarely ever saw snow/ice. Their campus had a dozen different buildings, all 4-16 stories tall and connected by skyways. The skyways made a massive difference and made life significantly easier.
The obvious benefit is that they provided protection against rain. Some less-obvious benefits:
Could be justified in any area with a lot of tall buildings, where a lot of people want to get from one building to another without having to travel all the way to the ground and all the way up the other side.
But as an alternative answer, a powerful Skyway developer happened to have politicians in his pocket or powerful lobbyists and managed to convince the right people that the city needed them (whether or not it actually does)
There is a very simple answer to this that I didn't see mentioned anywhere else, so I thought I'd add it even though an answer has already been accepted. The city I live in (Cincinnati, Ohio) used to have a very extensive Skywalk or Skyway. From the 1960's through the 1980's indoor shopping malls became very popular all over the United States. Traditional store fronts on the street simply went out of style in favor of air conditioned indoor shopping malls. Cities that wanted to capitalize upon this trend started building Skywalks/Skyways throughout their cities so they could compete with the malls at attracting both retail and food service businesses as well as the customers. Today the Skyways are less popular and cities that don't have a good reason to keep them have been removing them because the trend has gone back to strip malls and street front stores while the indoor shopping malls are in decline.
Your city has a warm nice climate... Too bad that the local ecology is basically nonexistent since it's in a quite industrialized area combined with bad local geographical features amplifying the problems. You REALLY don't want to get soaked in local rain, and some smog variations crop up from time to time as well. Skyways help reduce your exposure to outside conditions.
Tall skyscrapers are just a thing in your world. Be it residential, offices, educational, and so on, and so on; everything is a skyscraper!
Naturally, the rich wants to be in the highest positions available. They can see the view (though perhaps I imagine it's only natural background overlaid with views of spikes of concrete tops on the lower half field of vision). They want to be symbolically higher and more esteemed than the commoners. They want sunlight, because in the lower levels the other buildings block it from your windows. They want fresher air when they open their windows because ground level air just sucks (it's so humid and damp, ew) and is full of unpleasant motor smells.
The next thing they want is to avoid ground level as much as possible.
They want their kids to enter the prestigious school next building but don't want the hassle of descending so far to the street level just to cross and to ascend again. So skyways are constructed.
They want to visit their fellow established members of society but don't want to meet the commoners. So skyways are constructed.
The streets are dangerous because there resides the poor, the disadvantaged, and those who have nothing to lose when they commit crimes, so skyways are constructed.
Soon after, the middle class who resides in a bit lower stories than the rich wants to have the skyway for the same reasons. They distance themselves from the peasants that live near the street.
Voila, your city now has skyways for very sinister reasons.
Hundreds of years ago, your city was founded in the northern prairie. The gateway to the northwest, it thrived - but in the winter, temperatures rarely rose above freezing, often falling between -15C and -5C. To deal with these cold temperatures, the city built many skyways between its downtown buildings, allowing its citizens to cross between them in safety and even comfort during the long dark cold winters.
As the city grew, so did the rest of its civilization, discovering fossil fuel deposits and combusting them to fuel its economy. Over the centuries the average temperature warmed, changing local climates all across the planet. People moved from less comfortable places to more comfortable places, and as the temperature inched upward, your city thrived. As the gateway to the northwest, your city was positioned between the old great cities of the south and new farmland, resources, and trade routes unlocked from the thawing tundra.
Now the long dark winters are decidedly less cold; the temperature rarely falls below freezing, usually between 2C and 12C. People stroll about in comfort and when there is a dusting of snow, highways clog, schools close, and everybody generally loses their minds.
But the hallmark of the city's ancient architecture remain. Long a symbol of your city, both the old skyways and the newer ones have been meticulously maintained. Though they are often built open to the air now, with cool breezes blowing through and people often taking dinner on the wide new ones, the skyways still allow people to walk in comfort from skyscraper to skyscraper in the city core.
It's a niche application, but you will often find skyways linking courthouses to the county jail (not prison).
Reduces risk of prisoners escaping while being moved to/from court appearances since there is no external ground transport involved-- it's a literal pipeline.
Climate is not a factor; both Seattle and Atlanta's courthouses have skyways, as does the Doge's Palace in Venice (famously known as the "Bridge of Sighs").
These could be chemical, biological, nuclear or rogue nanotech floating around the atmosphere and commuters might want to keep their exposure to these agents as reduced as possible
Perhaps the scenario is a post-apocalyptic, post-war or cyberpunk type of environment where there is a strong preference from a land faction toward guarding from satellite or environmental observers
The only thing I really have to add that wasn't in the previous sections is that in minecraft skyways are more common. this is because not having to terraform the ground saves time (and creating structures that would otherwise be ludicrously unstable are not due to the rarity of block gravity) making it so building in such a way spends less time, which is compounded further when in the late game flying becomes available.
In short civil engineering is hard and terraforming takes time, so that if you can avoid the latter easier than doing the work then it's go time.
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