On many planes the overhead bin door is convex and the dimensions of the bin are such that if you but your rollaboard into the space with the wheels in you won't be able to close the door (as the bottom edge of the door and the bag get into a fight). If you put a pillow under the front edge of your bag (the edge closest to the aisle) you'll raise it up and it will fit into the convex area of the door... and more importantly the door will close.

This is most useful on 737s and 757s.