Once Samus goes into morph ball mode, she and her armor will shrink to the size of a small ball, and will then be able to travel that way, fit through tight paces, lay bombs, travel up walls if they have a spider-track, boost, and do other things as well. Usually in games she'll either 1. have to find it at the beginning of the game or 2. have it at the beginning of the game, though quickly loose it, requiring you to go back out and find it again. This is not the case in Metroid Prime 3: Corruption, however, where you never lost the ability.