Re: I wonder
The more obvious one is you'd like to paste the face of your big-name, impossible-to-insure actor onto the backflipping-out-of-a-thirtieth-floor-window body of your no-name stuntman. They've been doing exactly this with varying degrees of convincingness since at least Jurassic Park (they pasted the girl's face onto the stuntwoman doubling for her when she was hanging down into the kitchen where the raptors were).
Another: you've just shot a movie at enormous expense using a big name highly decorated actor. The last reel of film is in the can (or whatever the digital equivalent of that is), and just as you go into the edit suite it turns out your star has been fiddling with kids his whole career. Do you
(a) hire a different big name actor, remount the shoot and do the whole thing all over again? OR
(b) hire a different big name actor, take a couple of hundred photos of their face, and simply paste it over the performance you already paid for?