Why do people clap at the end of a movie?

I've never been able to figure this out.  The act of applause, at least as I understand it, is to show appreciation for an accomplishment.  That's fine, I'm not saying the actors and crew of a movie shouldn't be recognized for their work, but it's silly to clap for them when they're not there.  Applaud them at the premier, where a good chunk of the crew will be present; applause in this situation is fine, encouraged (if the movie was worth it), and likely expected.  But clapping after seeing a movie down at the local cineplex on a Saturday night?  Are you recognizing how well the film was threaded through the projector?  How neatly the ticket stub was torn?  How well the butter was distributed on your popcorn?  While it's true those are good skills to have (if you work at the theater), and they are all important parts of your movie-going experience, they have nothing to do with the movie; you won't see the film threader, ticket taker, or popcorn butterer listed after gaffer, best boy, and key grip.