Just Wondering: Why do celebrities use fake names at hotels

I remember watching the movie "Notting Hill," in which Julia Roberts' character checks in under fake names.  After that, as I became more and more celebrity-gossip-obsessed, I realized that the famous actually do this, which makes me wonder:  why?  What do they get out of it?

It doesn't seem to help avoid notice.  The paparazzi have absolutely no trouble finding the stars, so far as I can tell. 

 Is it to avoid getting phone calls?  Why would that be necessary, if nobody knows that the actor, singer, or whoever, is at the hotel?  Do people just randomly call hotels and say "Put me through to Mr. Cruise's room, please?" and the hotel does it?  Why wouldn't the stars just say "no calls, please," and then expect friends or relatives to call on their cell phone?

The way I see it, there's a few things that could happen when someone famous checks into the hotel:  either nobody notices, or somebody notices.  If nobody notices, using a fake name isn't necessary because nobody knows that person is there. If, on the other hand, somebody does notice, checking in under "Fred Flintsonte" wouldn't fool anybody because someone saw the celeb checking in.

 So why do they do it?