There is a lot of misunderstanding about this because of incorrect internet information. The size of a low bass wave means it can not be absorbed by carpet. Its mostly for decoupling the subwoofer from the floor, however there are people who may have such thick carpet that may interferes with the drivers travel and clearance spec. That in most cases is an extreme case.
The decoupling the spikes/feet prevents any spurious vibratory noises that may occur. You could also use soft rubber chair stops or felt pads under each foot/spike.