Image source Larger helicopters have landing wheels, and as we can see on the picture above, they are placed as far outward as practically possible in order to provide stability in the landing.
Touch down one wheel on an sideways inclined slope, lower the collective, and the helicopter will align itself onto the slope with: The location of the Centre of Gravity (CoG) always in between the wheels.
The higher the CoG, the wider the landing gear impact points must be.
The weight being supported on clearly defined and structurally reinforced points.
Even if the bottom of the helicopter is perfectly flat, the landing ground may not be, and to support all of the weight at any random point at the bottom requires a lot of relatively useless weight for the re-inforcement.
The behaviour after first wheel touchdown being clearly defined.
The wheel is a point of impact upon the landing plane, mathematically much more defined than a plane of impact upon a landing plane if the latter is curved.
All the above is for wheels, which allow the helicopter to taxi on-ground.
Skids perform all of the above functions apart from the on-ground taxiing, and are cheaper and lighter than retractable landing wheels.
Of course, the other downside is that they have more drag.
One would not want to land this helicopter without the skids!
aviation.stackexchange.com/users/21091/Koyovis
Physically they likely could, but helicopters (and all aircraft for that matter) tend to have stuff mounted to the bottom like antennas, probes, cameras etc.
so adding some kind of landing gear or skid helps to provide clearance for all that.
This question covers a bit about whats mounted where.
The belly of a helicopter also tends to not be perfectly flat (depending on the model)
so belly landing can lead to some roll which may allow the rotors to scrape the ground.
aviation.stackexchange.com/users/3677/Dave
Skids are especially beneficial when the helicopter is supposed to land on rough ground, e.g. grass.
They move the body and the rotors farther away from the ground, thus giving both more clearance.
Compared to extending the body to the ground to achive the same effect, skids add less weight and significantly less air resistance.
Compared to wheels they are better suited for rough ground, as they have a larger contact area and thus tend to sink less into the ground.
Wheels on the other hand have the advantage when the helicopter usually lands on concrete or other solid ground.
Here they allow taxiing.
That's why you tend to see skids on smaller helicopters which tend to land on grass or other uneven terrain, while larger helicopters, which tend to land on more solid ground, tend to have wheels.
Also, skids are simpler than wheels, which also suits smaller, cheaper helicopters better.
aviation.stackexchange.com/users/24599/Dakkaron
Skids, as opposed to wheels, allows the pilot to get some sense of balancing tail rotor counter torque before lift off.
Skids would provide more drag until the pilot "got it".
Same thing moving forward, backwards, or side to side.
The skid friction limits movement (or damps it if you will), rather than creating a nightmare of correct/overcorrect in 2 dimensions. Skids also add an important amount of rotor clearance, and offer simplicity, reliability, and light weight.
aviation.stackexchange.com/users/32094/Robert DiGiovanni
An additional benefit of longer skids is some energy absorption upon a vertical crash.
The skids are generally designed to bend outwards and up during impact.
This can be seen occurring on many helicopter crash videos.
The design of impact absorbing skids is covered extensively here .
aviation.stackexchange.com/users/43893/johnDanger
As pointed out above, not all helicopters use skid style landing gear.
Some have either a fixed or retractable wheeled undercarriage.
These offer excellent mobility on paved or short cut grass airfields, surface taxiing, rolling takeoffs or landings or for movement aboard ship.
Surface taxi also consumes a little less fuel than hover taxi or takeoff from an IGE hover, but are more complicated, heavier, and expensive than skid type landing gear.
Smaller, utility helicopters usually use a skid type landing gear For several reasons.
Lightweight.
Inexpensive.
Flexible and can absorb impact of a hard landing or crumple to absorb energy.
Easy to replace if damaged.
Provides a large, stable support for pinnacle or slope landings.
More even distribution of aircraft load for landings in soft or waterlogged fields.
Lends itself better to being fitted with fixed or emergency inflatable floats for over water operations.
Can provide support for personnel flying outside the airframe.
The downsides to skids are
Do not allow the helicopter to be easily moved or towed on the ground; must be fitted with towing dollies or placed on towing trailer.
Surface movement limited to hover or air taxi.
Are fixed and cannot be retracted and faired, thus adding more drag in flight.
Easier to snag on obstacles and wires during surface operations.
More susceptible to side loads during takeoff and landing, which can cause dynamic rollovers.
Great skill must be used during rolling takeoffs, shallow descent landings or autorotation to keep the aircraft moving in a straight line down the runway to prevent side loads.
The aircraft’s fuselage is shaped primarily to provide the best aerodynamic fairing for the cabin, engine and transmission and aircraft systems; it is not necessarily the most stable platform and does not provide any shock absorption in case of a hard landing or an autorotation landing.
Landing gear is designed to solve that problem.
The siding of the skids is based on a lot of things from the ability to provide solid, stable support on uneven terrain to prevent the onset of potentially deadly ground resonance during a harder than normal landing to offering good ground clearance.
aviation.stackexchange.com/users/15637/Carlo Felicione
Why can't they just land on the helicopter's floor? If it's for stability, they could still use skids which are shorter than what is commonly used.
Why do helicopters have skids?
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<helicopter>
aviation.stackexchange.com/questions/73842
aviation.stackexchange.com/users/9329/Ronen Festinger
Helicopters have skids because they absorb the impact on landing. They also provide more stable landing on difficult terrain. Moreover, as helicopters often have equipment fitted to the outside, e.g. cameras, the skids can be used for this. A further advantage is they can be fitted with floats for activities over water. However, not all helicopters use skid-style landing gear. There are many advantages to skids: they are ideal for landing on rough surfaces, they are light, inexpensive, flexible and can absorb impact, easily replaceable, can be fitted to land on water, provide easier landing and more room for equipment appended to the underside of the helicopter and support for personnel. A landing on their belly is often problematic because the belly is not perfectly flat and can often lead to some roll and possibly endanger the helicopter's equipment. Even if it is perfectly flat, the surface on which it has to land will most probably not be. That said, there are in fact helicopters that do not use skids to land.
There are many advantages to skids: they are ideal for landing on rough surfaces, they are light, inexpensive, flexible and can absorb impact, easily replaceable, can be fitted to land on water, provide easier landing and more room for equipment appended to the underside of the helicopter and support for personnel. A landing on their belly is often problematic because the belly is not perfectly flat and can often lead to some roll and possibly endanger the helicopter's equipment. Even if it is perfectly flat, the surface on which it has to land will most probably not be. There are helicopters that do not use skids to land.