The ball-head lets you go in at a slight angle, a real blessing when mounting racks or bottle cages. Actually, "a real blessing" doesn't go far enough. Mounting a single bottle cage without one of these will drive you insane.
Sometimes you need a short arm allen to get in place. That's why you need some of these in addition to the Bondhus multi-tool we also sell.
3mm x 100mm - For little bolts, some brakes and lights and other teeny things.
4mm x 112mm - Bottle cage and rack bolts.
5mm x 121mm - Most popular size, for stems, handlebars, brake levers, shifters, racks, all over the bike.
6mm x 140mm - For stems and the seatpost.
8mm x 163mm - For allen style crank bolts.