A disk is a problematical beast. by default, all the sanding is cross grain, and it will tend to cut a lot harder on the leading (high) edge. What do I use the belt for? everything., it is hard to be specific. justgeneral shop chores. One other thing, a belt will tend to last a lot longer than a disk, as it stays a lot coole, and the grit changes are much easier, as a disk is generaly glued down.