Supply Crates are very common in the game and can be found almost everywhere. The main purpose of the Supply Crates is to restore the ammo for the Kong's weapons. When a Supply Crate is picked up, it will add 10 ammos to the current ammo supply.
There are also red Supply Crates that contain homing ammo. However, unlike the regular Supply Crates, the homing Supply Crates only adds 5 ammos to the supply and they are much more rarer in the game.