why do some turntables have multiple tonearms?

If you collect records and like to have the original artifacts, you'll need to play 78s with a 2.7-3.0 mil stylus, a mono configuration and most likely a heavier tracking force. If you enjoy early mono LPs played back properly, you also need a mono setup and a 1.0 mil stylus. This is in addition to a stereo rig with a 0.7 mil (or thereabouts) stylus. You could have three turntables but the easier (and more space-saving) way to accomplish this would be to have three arms on a single turntable with switchable speeds. The next best solution is a single arm with three removable and weighted headshells/pickups, but that takes more fiddling around when you just want to listen to records.  

Interesting to note that this thread lay dormant for seven years.  Maybe because the possible responses are so obvious.  The OP's question is almost in the nature of a troll.  Anyway, my response is "Duh!".