Fillers and spacers really elevate your kitchen to give it that custom look you want.
Fillers are 10" wide. They are intended to fill in spaces, especially between cabinet boxes and walls. IKEA recommends a minimum of 2" between your cabinet box and the wall, so that the doors can open all the way. For example, if your cabinet is installed 2" away from the wall, and you want to close off that space, you would use a filler.
Fillers are 10" wide because of how they need to be installed. Your installer will actually cut the filler into two pieces. One piece of your filler will have then two edges: the finished edge, which goes up against your cabinet, and the unfinished edge from your recent cut. The unfinished side can then be scribed to fit against your wall perfectly. The other piece of your filler is used as a supportive backing. It also makes sure the visible piece sits flush with the cabinet face next to it.