All real military ground vehicles use a continuous caterpillar track or tank tread, but quad-treads/split tracks are often seen in sci-fi, with their futuristic tanks and settings. (Halo scorpion, C&C mammoth etc)
Is there an actual reason(s) or advantage(s) to using a quad-tread/split track?
Or is it only a common, impractical, artistic design choice?
[Edit; Answer supplementary]
There are indeed some advantages to having multiple treads.
One big advantage is in the geometry. With two treads, the tank's pitch (the angle between the vehicle's nose and the horizontal) is on a line tangent to the ground underneath the vehicle's center of mass. On mostly level ground, this means nothing. When the ground changes elevation suddenly, though, it can become a problem. As an example, here's what a two-tread tank looks like coming over a hill.
The tank's pitch is still at an angle as if it was on the hillside, even though the ground has already flattened out. This has a number of problems. The tank has limited traction with half its treads in midair. The less-armored underbelly is exposed to enemy fire. The driver is looking at the sky and can't see what's directly in front of him. The main cannon can't adjust very much in the vertical direction, so large changes in pitch have a major impact on where you can and cannot fire. After the tank's center of mass crests the hill, the front end comes slamming down on the ground. That's not good for your suspension, and even worse for the poor occupants who are suddenly thrown 8-10 feet downward while stuck in a cramped metal can. These geometric issues grow worse as tanks grow larger and the distance between their center of mass and the end of the tank increases.
If instead you had four treads instead of two, you can handle terrain changes much better. Each of the four treads can be at a different angle. This sets the tank's pitch roughly equal to the average of the pitches of the individual treads (the geometry is actually a lot more complicated than that, but it's close enough for discussion purposes). In the photo above, a four-treaded tank would have its front treads flat on the hilltop and the body of the tank would be at half the angle shown. Instead of slamming down after cresting the hill, the tank would gradually level out since all four treads would remain in contact with the ground the entire time.
There is a drawback here, though. Imagine in that photo that the hill was shaped like an inverted 'V'. After the tank topped the hill, it immediately faced a steep downward slope. A four-treaded tank could potentially get stuck high-centered on the top of such a hill, the same as a truck might do. A two-treaded tank can't get high-centered so it would continue unimpeded. It would be an absurdly unpleasant ride for anyone inside, but it wouldn't get stuck.
Another potential advantage I've heard of for multi-treaded vehicles is regarding tread wear. The wheels at the front and back corners keep the tread in tension. Even as far back as da Vinci, it was known that a shorter length of cable can generally withstand more tensile stress than a longer cable. Having several smaller treads means less tread is held in tension, resulting in fewer stress-related failures. Some of the photos in the other answers show an additional top wheel that makes the tread follow a triangle pattern, further reducing the amount of tread held in tension.
Another advantage is that tracked vehicles have a separate motor for each tread. As tanks grow in size, they will at some point become so heavy that there's no practical way to propel them using only two motors (the motors become too big for the chassis, or they require more fuel than you can carry). Having more treads means you can have more motors. Each of those motors can be much smaller than the motors in a two-treaded vehicle. This design can be cheaper, more fuel-efficient, and can deliver more overall power. Many wheeled vehicles designed for pulling extreme loads (like the Soviet MAZ-7907) use a similar one-motor-per-wheel approach in order to generate enough power.
It's worth noting that one of the proposed Mars rovers (for the Pathfinder mission in 1995) was a quad-track:
I haven't been able to find out much about this rover design, so if anyone knows who built it, or has more information, I'd really like to know more. Rather curiously you'll notice that this particular machine doesn't have any idlers or boogies under it's tracks. That's because it's an ELMS (elastic loop mobility system) using a titanium band as a track.....
Another quad-track is the tuckers sno-cat:
So this sort of vehicle definitely does exist. Why are two-track vehicles more common? Here are a couple thoughts:
However, as a builder of micro RC tracked vehicles, I can tell you that a quad-track would be much better at dealing with obstacles such as fallen logs and steps. A two-track vehicle tends to have it's nose ride up until it pitches over backwards. A quad-track of equivalent size would have the vehicle move horizontally in situations where a two-track would pitch over backwards:
So what can we do to mitagate some of the issues and end up with a setting where you can have a quad-track?
Well, a lot of the problems go away if you have a [fuel cell, fusion reactor, handwavium electrical source]. This means you can eliminate all the drive-train issues. Current vehicles can't do this because introducing high-power-hybrid-drive-systems to a military vehicle is unlikely to increase reliability, but if a non-petrol-generator electrical system can be found....
Redundancy. A tread is a single body, and if it's severed, the entire drive unit is useless. If that's your only drive unit on one side, all you can do is spin in circles and you're done for. If you still have one active drive unit on that side, you may have some mobility remaining to retreat or regain formation with your allies. (The Spirit rover is an interesting example of multiple independent wheels that allowed it to continue operating despite the complete failure of one of its wheels; the principle is similar for independent treads.)
Independent suspension. A vehicle with multiple, independently-articulated treads can keep them all in contact with the ground over a much wider variety of profiles than a single pair of longer tracks. For a larger vehicle, as many sci-fi tanks tend to be, this might be essential to maneuver effectively in the same terrain as modern-sized tanks. One or both pairs can articulate sideways as mentioned in EstimatorNoiseless's answer for better maneuverability.
Modularity. In a sci-fi setting where you may be establishing operations on a frontier planet or in a deserted wasteland, space in factories and warehouses is at a premium. Being able to reuse drive units and treads between different-sized vehicles could benefit your supply chain.
Sheer size. The very largest treaded vehicles in the real world - things like the crawler-transporters that carried complete Saturn Vs and space shuttles out to the launchpad - often have more than two treads. Bagger 228, an excavator, has twelve separate tread sections mounted in three groups of four. Presumably, at these scales, some combination of the weight of the tracks (harder to get them to turn) and their physical dimensions (harder to find a factory with machines big enough to make them) makes two-tread designs impractical. Of course, in the real world there's no particular military use for a tank of this size, but that's never stopped sci-fi.
There is one other niche purpose for large split track vehicles. They are fairly common in large agricultural tractors. Track use in large tractors has come about for the same purpose as load distribution mentioned in ksbes answer. Tilled soil can be soft and loose, but it is also in the farmer's interest to avoid compaction of the soils (which can adversely affect the productivity of their fields). More surface area means less soil compaction as the tractor moves across it. Having quad tracks allows the tractor to maximize pulled loads as well (especially over uneven ground), as increased surface area will increase the traction. That said, there's a trade off of maintenance costs which are typically much higher on tracked vehicles over tires.
I drive one of these quad-tracked tractors. When carrying a lifeboat the total weight is over 56,000kg and it is able to do 16km/h over flat ground with that load. The suspension has no springs, but due to the vehicles articulation and all four tracks being hydraulically powered and able to independently tilt to match the terrain, it is amazingly capable for its 20m length!
The four tracks also increase the surface area in contact with the beach, so despite its weight, the vehicle can drive over soft sand without bogging down.
For those that are interested, here is a video showing what this vehicle is used for: https://www.youtube.com/watch?v=MZ75LIOavbs&t=70s
Like other answers mention, if the vehicle only had two tracks it would rock dangerously when traversing hills or undulating terrain. The four tracks conform to the surface much better.
Towards the end of the Soviet Union, there were many experimental tank and AFV designs in an attempt to gain decisive superiority over rapidly advancing Western AFV's. One common feature in these designs was the proposal to use 152mm cannon to deliver massive amounts of kinetic energy against even heavily armoured Western tanks carrying composite armour, as well as a huge HE or HEAT shell for fire support against unarmoured or lightly armoured targets.
The ultimate evolution was the Objekt 490. This was a very low vehicle mounting a 152mm cannon in a limited traverse turret. The 4 track system did not provide the articulation advantages that you might find on a tractor, since the chassis was a single unit, but the arrangement allowed the use of two separate engines (road marches could be conducted with one engine off for better fuel economy), lower ground pressure and the ability to continue moving should two sets of tracks be damaged (one on each side, obviously). You can see the layout in these illustrations
Objekt 490 layout
Objekt 490 side view
Objekt 490 rear view. The gun is fully traversed to the rear, and thus impossible to fire
Sweden also did some experimental work at the same time, seeking to replace the Striv 103 "S" Tank. One idea investigated was the UDES-XX-20, which essentially expanded the idea of the BV-206 articulated marginal terrain vehicle into a fully armoured fighting machine. For the Swedish terrain, the low ground pressure and articulation would have been very useful, but expensive and mechanically complex. The Swedish Army and government eventually decided the costs of creating a unique "Swedish" tank far exceeded the benefits, and Sweden went on to adopt a version of the Leopard 2 instead.
UDES-XX 20 general arrangment
UDES-XX 20 demonstrating articulation
UDES-XX 20 climbing a slope
AS you can see, while there are some advantages to multi track designs, for AFV's the added complexity really comes at too much of a cost for the advantages that you get. For current tanks and the foreseeable future, multi track designs will likely only appear on marginal terrain vehicles where the advantages are very clear and unachievable by other means.
Royal Marine Commandos in Viking MTV's
It is quite impractical for all-terrain vehicle.
What is tread is used for? - to support high load on soft surface. It means one of two things
-OR-
There is only one exception - extreamly soft surface. It is either snow (and even there half-ski-half-tread is preferable), or any surface for thousands-ton vehicle. But this exception happen due to requirment of destributing mean load over even larger area (to prevent surface sliding) and (for super-heavies) relative softnes of construction elements.
So for military vehicle qard/split-treaded is quite impractical. There were experiments with split tracks for heavy tanks in USSR. And they showed all the flaws of that idea.
Robots like packbot use two sets of treads. One is stationary on the body the other set is mounted on rotating flippers which is useful for overcoming obstacles.
