Commercials are the best part of being a celebrity. Show up for a few hours, get paid hundreds of thousands of dollars. Really the biggest upside of being famous.
It's also the only steady paycheck you can hope for in that kind of career. It's actually quite lucrative, especially if you are lucky to get on a long-running campaign--like Peter Groszc and TJ Jagadowski--those dudes that have been doing the Sonic ads for...forever now. I recall seeing them years ago doing improv in Chicago (they are kinda big in the improv world), and that ad campaign is pretty much what they do now, aside from the occasional bits in the various other improv-specialty movies that feature that staple crew, and I'm guessing the guest spots at various 2nd City shows wherever they are--that kind of thing pays basically nothing, though. Those ads pretty much suck now, but those two dudes make absolute bank doing it, and I think a 5-ad set takes something like 1 day of work, and can run for 1-3 years.
I knew a guy that used to be that "Nintendo Gamecube guy"--there were maybe 6 or so ads overall, but he played an employee that was a bit of an idiot, stocking Gamecubes, doing...whatever stupid stuff, working in a toy store. I think that ran for about 3 years. It was is only real paycheck, aside from various small gigs here and there, and he did pretty well with it for that time.