That's mainly for his security, and for the purposes of a buy out if he starts sucking. They'll have to pay him ridiculous money to go away in a few years if he just lays down and quits working.
He won't do that, but that's all it does. If he takes them to another Final Four or keeps having success, Butler is going to have to amend the contract and give him a raise or two and an extension, and he can still leave whenever he wants. I would imagine given his negotiating position that his buy out of the contract is pretty low, and if he has more success, any school that wants him would foot the bill on whatever his buyout is.