Butters is like an inverse of Luccio. She knows the theory of computers, could probably write out an algorithm to sort a file efficiently, for example, but is unable to put that into practice because she'd roast any computer upon which she tried to program.
Butters, likewise, could probably design an efficient spell, but doesn't have the juice to make it work, his theory is only theory, he needs someone else, like Bob, to put it into practice.