Anytime you have a run of cabinets that dies off into a wall, you would use a filler to effectively space the cabinet away from the wall to ensure doors and drawers can function properly. The walls of your home will be off a little in terms of being plumb and level. The filler will act as a transition piece from an imperfect wall to a perfectly square cabinet. In addition, fillers are used to allow the doors to operate properly without rubbing the wall, as well as make the cabinet look more "built in", and professional.