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 PratchettThey always gives me bath salts," complained Nobby. "And bath soap and bubble bath and herbal bath lumps and tons of bath stuff and I can't think why, 'cos it's not as if I hardly ever has a bath. You'd think they'd take the hint, wouldn't you?
Terry Pratchett