Before Mission Control, there was Exposé. It was flexible, it was novel, it was a little confusing. In Lion, Apple incorporated the main functions of Exposé into Mission Control—but Apple didn’t want to break all those Exposé fans’ hearts. So in Lion, Exposé is still there, for the benefit of old-timers who don’t like change.
Well, it’s mostly there. There used to be three different Exposé modes. One showed you miniatures of all windows in all programs; one showed you miniatures of all windows in just one program; and one hid all windows in all programs so you could see the desktop.
In Lion, only the second and third Exposé functions still exist. (The first one—showing all windows in all programs—is what Mission Control is for.)