This feature pushes filament into, or pulls it out from, the hot end. It helps to swap the filament, for instance, when changing colors. The extra filament can be fed in to get the color just right.
Also, this function allows us to quickly check if the e-Steps are calibrated properly and if the right amount of filament is being fed in. For example: marking 120mm on filament and then asking the machine to Feed In 100mm and then measure if the amount was correct. Full 二次元美少女 guide is here.