Why are you making war in the first place? Your reasons dictate your methods, because the costs aren't always acceptable. Usually, though, you're not fighting a war of extermination: I'm going to be running on the assumption that you're referring predominantly to wars of conquest, with somewhat more nuanced objectives than "kill them all".
One reason for ground forces is that you need them to, well, hold ground. You can't effectively govern a city entirely from orbit, even if you get them to surrender somehow. You need to enforce curfews and martial law and so on, whatever policies are necessary to maintain order, and you need boots on the ground to do that. If you stick to just orbital surveillance, you are guaranteed to miss most dissident action, subterfuge, etc.; in other words, you don't actually control the territory at all.
Reason number two: in a war of conquest, you want to acquire something the other side has, typically territory and/or resources. Those wide fertile fields of Planet Farmerville aren't going to grow any crops after they've been reduced to wastelands by bombardment. Those valuable mines on Planet Minertown, full of fragile unobtanium that's worth twenty million credits a kilogram, collapsed by your nuclear bombs? Great job denying the enemy that resource, but you're cutting off your nose to spite your face at that point, since you've now destroyed your reason for trying to take the planet over in the first place.
Reason three has to do with precision. I already touched on this with how bombardment will tend to pulverize the very resources you're trying to claim, but it goes beyond that. If your standard approach to warfare is to blast apart all opposition from orbit, you will be killing a lot of civilians along with the soldiers. You're going to turn the survivors into a massive guerrilla force: even if you win, the ruins left behind aren't going to be anywhere near profitable or useful enough to justify the cost in blood and gold. That sort of massacre is also going to turn others against you in the political arena; it's going to escalate the war, and see similar retribution against your own people. This is basically why the laws of war were invented: they help keep wars from spiraling into the sort of Pyrrhic disasters where both sides lose.
I also see you wondering why beachheads are necessary as a specific example. This one is a fairly simple problem in concept, even if the reality is something military professionals are forever balancing: numbers and cost. Why didn't the Allies just deploy an army entirely of paratroopers in the D-Day invasion and bypass the bloody amphibious beach assaults altogether? Drop pod tactics (and rigors: riding in one of those is going to be much harsher on the troops than a transport) are for elite soldiers. You can't feasibly have your entire army trained to the level of special forces: the bar is a lot higher than for regular troops, which reduces your total army size, not to mention the additional cost of training and equipment for those specialists. Elite soldiers have their place (drop pods would be excellent for disrupting lines), but they can only do so much if they're outnumbered six to one by the other side going for regular troops instead.
Also, troop transports are going to be a lot cheaper for troop deployment than drop pods. Using transports will net you far more troops on the ground overall than drop pods; you sacrifice some flexibility in deployment, but both have their roles. Using dropships or shuttles rather than drop pods means you don't need as many vessels dedicated to transportation of forces (or as much space on your existing vessels, if they're all multi-purpose warships). Keeping costs down means more warships or ground forces can be built/trained and outfitted, which also helps raise your numbers.
As for the beachheads specifically, drop pods and similar tactics are not practical for resupply purposes: they're hideously inefficient for providing extra ammunition or other war materiel and don't offer a means to retrieve the wounded, reposition soldiers if a local retreat is needed, etc., so you need those transports regardless. However, transports are usually underarmed and underprotected (hybrid transport/combat-armed shuttles are a lot more expensive and typically less effective than specialized vessels), so they need safe places to land: beachheads. Major targets are probably well guarded by anti-aircraft fire, not all of which can feasibly be removed by targeted orbital strikes. Conventional two-dimensional war maps with front lines are a stretch, but on densely populated planets with numerous well-defended points of interest and a strong defense force, they might happen, especially if there are any ground-based defenses powerful enough to reach orbit and thus limit or deny any of your forces in the air or space near those areas.
Conclusion: You need ground forces to take control of territory, to capture its resources and put them to your own use. Bombing might be the most efficient and least costly means of killing, but warfare (usually) has some other objective than genocide or mass slaughter, and the collateral damage will wreck the value of whatever you've conquered.