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!