why do some turntables have multiple tonearms?

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.

Use the second tonearm to check the condition of the newly bought used lps.

well, normally you use different cartridges for 50ties and 60ties music and again different for the after 7ties pressings....(SPU MONO65, SPU MONO25, SPU GE and SPU Silver Meister) and of course the EMT TSD 15.....FUN!

The older I get, the more I appreciate mono recordings. Having two tonearms allows for dedicated stereo and mono setups at the ready. It makes things that much easier.