Why create derivative works?

Shannon Hale, cocreator of Rapunzel's Revenge, explains her reasons in an interview at Girls Read Comics:

With Rapunzel’s Revenge, the fun is taking a well-known fairy tale and unraveling it. The advantage of this instead of just writing an completely original story is the Grimms’ version is in constant dialog with ours, asking questions and creating complications that wouldn’t be there otherwise. It’s also satisfying on a personal level, because Rapunzel is to me the most irritating fairy tale of all time and I couldn’t let it stand without having a snarky thing or two to say about it.