Students with muggle parents have to exchange their money for wizard money but why would wizards ever have use for muggle money?

Well, even they need muggle clothing- like the Quidditch Cup thing. Wizards usually have robes on but like I said, they sometimes need muggle clothing.
They may need it for food, since there is that Gamp Law thing.
For travel- taxis like when the Weasleys and all that went to the station and the taxi drivers were annoyed because of the owls.
Things like that.