why do some turntables have multiple tonearms?

have several different cartridges available to choose from without having to go through the hassle of alignment each time you want a change (different flavour). or to have a stereo and a mono cartridge already setup for use.

If I ever treat myself to a VPI HRX, Clearaudio Statement or Goldmund Reference II with more than one tonearm, I don't think I am going to spend a lot of time with mono records. But maybe its just me.

Like a lot of very expensive aspects to our hobby, its probably just for fun.

Each set up will have a distinct "signature". Great if you can afford it!

Great way to test the sound of various cartridges and arms, provided you're patient enough to own them for a few months and do serious listening.

I have two tables, four arms and five cartridges. It really puts things into perspective to have each ready when you have a great LP spinning and can immediately confirm what each rig does.

In my case the two tables are absolutely identical, down to (at least two) matching tonearms and all matching phono cables. My phono stage allows load and gain on the fly and has four inputs. That helps a lot too, otherwise you loose momentum.

Some here at Audiogon have several times what I have. It's not uncommon for those of us that love vinyl records to have more than one arm.