Whenever a person is traveling, they do not think to take a towel or two with them. Choosing to provide towels to your guests will allow them the chance to shower throughout their stay without needing to venture out to the store to purchase one.
While many hostels provide towels at no charge, it is understandable if you want to charge a small fee. After all, towels have been known to disappear from hostels on occasion. Whatever you decide, you will want to make sure that the towels you are providing are of the highest quality and that you replace them often, so that they can still absorb water.