I think if you asked the Chantry why it was founded, it would say they wanted to end slavery and cast out the Imperium. But if you consider the reasons why the South wanted to end Imperial rule, I have to imagine that wanting to put a check on magical power was one of them, and probably a major one. When I say that the Chantry was founded to bring magic use under the control of a central authority, it's less about the official history and more about the practical outcome.