At the time I'm writing this, there's another writeup just above, which states that using more than two speakers in any sound system makes no sense, because humans don't have more than two ears. It goes on that digital signal processors are able to process sound in certain ways to make it sound like it comes from any arbitrary direction. In the final paragraph, it wants to make you believe that 5.1 systems at movie theatres are actually playing stereo sound and only pretend to have 5.1 channels.
Since this opinion doesn't originate from an appropriately qualified noder, I would like to put this straight. Please note that I'll explain nothing more than why it is incorrect to believe that using more speakers than you have ears would make no sense. I won't talk about the extra cost of installing hardware for more channels, problems with interfering sound waves, lack of media that take advantage of 5.1 channels, or anything else.
Firstly, digital signal processing can't truly emulate the way your ears alter sound coming from various angles, because every individual has slightly differently formed ears and thus would require a signal processor tailored especially to her to make the algorithm work perfectly. Secondly, you would need to wear headphones, because when the sound comes from a speaker, which is placed at a certain location, your actual ears alter the actual sound waves in addition to the signal processing, and your brain would still try to find out where it actually comes from (and it would succeed at doing so because of the point I'll mention next). Lastly, even if you had your personal signal processor and headphones, the illusion would be far from perfect, because when you locate the source of a sound, you turn and tilt your head around a little, observing the changes of the timbre caused by the filtering your auricles do.
Of course, there are good reasons why there are center speakers and other speakers lining the walls, but explaining that would be beyond the scope of this writeup -- just locate the appropriate entries. They'll also tell you what the digital signal processors in the signal line of, say, a THX set-up are good for. It has nothing to do with emulating 5.1 sound on stereo speakers. Or you can just turn around your head while watching a movie and you'll be able to verify that those sounds from off-screen sound sources are definitely coming from the surround speakers and not programmed to be perceived as coming from behind you.