If you remove the speech centers of the brain all at once, you completely and permanently lose the ability to speak or understand speech. Do the same but very slowly, and the brain is able to compensate, even if you completely lose your original speech centers in the process. NN with pruning is the same thing, where you prune iteratively while retraining to regain most of the lost regressions. If you prune too much all at once, you have to restart from scratch.