It's an old magical principle - it's even filtered down into RPG systems - that magic, while taking a lot of effort, can be 'stored' - in a staff, for example. No doubt a wizard spends a little time each day charging up his staff, although you go blind if you do it too much, of course.
Terry PratchettShe was convinced that she was anorexic, because every time she looked in the mirror she did indeed see a fat person.
Terry Pratchett