Well, In Kesha's (and other artist's) case, it's a chance for songs that were refused for the standard (Ie. Last Goodbye), or songs they wanted on the album, but couldn't because it didn't fit into the general flow of the standard, or because of restrictions to get those songs released properly. SOME artists, use it a format to make more money, while other's don't.
If you don't want to buy the deluxe edition, you don't have to. You're not being forced to get it? But I doubt any of you are going to skip out on buying the deluxe, IF she decides to do it.