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 by at least 2" (2" is the most common filler width). 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 into place.