Industry standard is the first reason to come to mind. Of course, if you're not in the "industry", that doesn't matter. What you "grew up" using is one of my reasons. I picked up PS around version 4 and used it since. I also have PSP (but only because it's so cheap I have both). I like a few things PSP does but I do 95% of my work in PS. With the new Bridge, the file browser in PSP will see even less use now. So, though a lot of things done in one proggie can be done in the other, I've just gotten used to how it's done in PS. As long as you're asking that question, you might ask why pay for either as long as Gimp is around.