"A lot of the same kinds of things that leaking, say, water might do. But magically. There are things, like paper, that if you get them wet they can be damaged or ruined; similarly, there's some kinds of magical things that are sensitive to leaked magic, and if they get too much on them they go wrong or stop working. And it's your magical power that you're leaking, so if you didn't leak, you'd have more of it to use for the things you want to do."