That scene in Iron Man where Tony is being marched around by terrorists and has a bag over his head, clutching a car battery to his chest? Someone in charge of casting realized "Hey, if we put a bag over his head, we don't have to pay Robert Downey Jr. to do this shot. Someone get Thrust Neckpunch over here!" The same thing happens later when a reporter hands him photos of Stark weapons being used to murder innocent people. When the camera pans down to show him flipping through the photos, those hands don't belong to Robert Downey Jr.
Thirty seconds of filming his hands costs more than most small businesses ever make.
It's also done for convenience, too. The camera angle required for the photo shot would've required Downey Jr. to stand bent over backward for hours at a time so the camera could fit where his head should be, and that's not something you subject an A-list actor to, even if you are paying him enough to actually become Iron Man in real life, should he so desire it.
Also, remember that the interior and exterior of a location in a movie may, in reality, be in different cities. In True Blood (where Thrust worked as an extra), the interior of the Fangtasia Bar is a sound stage, and the exterior is a real bar, miles away. So something as simple as a character walking into a building requires multiple shots and miles of travel. Well, you can't have the actors doing that shit, so you have the extra walk toward the building for the exterior shot, then cut to the star on the sound stage once he's inside.
Then you just FedEx the extra back to Los Angeles and keep right on filming.
So what looks like a single performance can, in reality, be the Frankensteinian creation of a whole bunch of actors and extras having their performances stitched together. Thrust said he once filmed a scene where he was a police officer frisking someone. He had no lines, but when he watched the finished scene later, his character had a line dubbed over the shot. Why? Well, it's actually cheaper to do things this way and pay two people -- the voice actor and the filmed actor -- because they can both be listed as "extras," rather than actors.