The bump stops are intended to provide a degree of "cushioning" once the suspension reaches the end of its travel and bottoms-out. In other words, if you were to totally remove the springs from the car, it would be resting on the bump stops. The idea of cutting the stops when installing a lowered suspension is to increase the amount of travel before hitting the stops (in order to try to compensate for the lost travel resulting from lower height).

Some bump stops are actually designed as a sort of " additional spring" (rather than a simple "stop) providing a added form of progressiveness to the spring rates in the last portion of the travel.