There are in excess of 175 conditions which are commonly treated with cannabis or medical marijuana. Not all of them are considered by the various states to be an acceptable justification for the use of medical marijuana.
Numerous patients have empirical evidence (first-hand experience) about its effectiveness. There is a fairly comprehensive (but by no means complete) list which may be found here. It can give you a good idea of just how useful this product is.
Typically the state-approved lists include the following: