why do some turntables have multiple tonearms?

i have seen some turntables that have 2 (or more) tonearms. what is the purpose for having more than one tonearm mounted on a turntable? what does it allow you to do that you can't do on a tonearm that has one tonearm?
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.
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.
- One for Stereo Cartridge, the other one for Mono
- one for this color, the other one for that color of cartridge
- one for Cartridge without, the other one for cartridge with body
- first one was not expensive enough, the other one is
- one 9" Arm and a 12" Arm
or the second one, because you don't trust the set up from the other one
or ... :-)
The battle cry of the militant audiophile: 2 ARMS! 2 ARMS!!! I tried it myself 30 years ago with a Micro Seki DDX 1000 but found that I used one to the exclusion of the others. Haven't tried it since and most of the reviewers I trust seem to rely on the single arm approach but there are certainly advantages to multiple arms; ask an octopus. Seriously, I am currently setting up a second table with a removable headshell arm so I can change cartridges fairly easily but I'm not sure how much I will use it. But since my phono stages have two separate inputs it seems a pity not to use them.
To me it is about the kind of music one is listen to. I use
Triplanar/ Benz Ruby 3s for vocals and the Reed 2A /Phase
Tech P-3G for piano music and symphonys. There are no perfect carts nor tonearms. So one should use what is best
in the given circumstances depending on the owned combos.

It is actually a great idea to have two different arms/cartridges for different music. Or three. Though expensive, it is in fact much cheaper than amassing thousands of records that you never or almost never listen because you have no time to. Unless that's the only thing you do.
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.  
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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!".