The oil-filled shocks and springs are the main parts of the suspension. The suspension helps to absorb the shock on rough roads and when landing after a jump. On professional RCs the suspension may include an anti-sway mechanism to keep the car from flipping on turns.
A three piece shock setup found on most toy and some beginner hobby RCs is one where there are only three pieces: upper, lower, and a spring of some kind. These shocks have no oil, rubber seals, or extra hardware etc. This type of suspension really doesn't have the ability to take a hard bashing and there are rarely any replacement parts if something should happen to break. This is especially the case for the toy-grade RCs. Kind of a "what you see is what you get."
On intermediate and high-end hobby-grade RCs the shocks and the entire suspension setup are not just for show. They play an integral role in how the RC handles and how well it performs under different terrain conditions. The suspension setup on these types of RCs as a whole also has a lot of variable options. This allows the owner a lot of leeway when it comes to tuning and tweaking. You can adjust things like ride height and rebound rate of the dampener shocks due to the type of oil used and the strength of the coil spring (which in some cases is indicated by color).