The obvious one being that you have to keep the story going without the character being on the metaphorical river in full flood, carrying him down a canyon. (Or a literal one -- that would definitely keep things moving.)
The other is that since there is a lot more room for random details, and furthermore necessity for it (to fill up the world and make it look real), there's a lot more room for things to siddle in, sit down, and announce they Will Be Important Later.
Mind you, it doesn't mean they have to help with keeping the story going. You have to invent a reason why a spell that creates carefully set to expire objects would be useful in the future. (To tell time -- actually even the muse managed to figure that one out quickly. It's when it's so crucial that matters. Or, of course, a sideways use in that it dissolves.)