What are charms used for on RuneScape?

Your question is a bit ambiguous, but I will endevour to answer your question. If you are referring to "What are charms used for?" then the answer is the summoning skill where you can summon pets (or familiars) to aid you in a variety of ways. To begin to train summoning you have to have completed the wolf whistle quest. A very useful and informative guide on summoning can be found on the runescape website under manuals.