Anything you want. Just because you have a certain thing, doesn't mean you need to be restricted to what people say it is used for. I use mine for rythm, lead, as tone control. Anything I can find a use for, I will use it. Just mess around with it, and you will probably find something interesting about it.

Listen to Jimi Hendrix or "White Room" by Cream, or "Sweet Child O'Mine" by Guns N' Roses. The Wah is used in the solos for both of those songs.