Why Would a Turntable Need More than 1 Arm ?

Ask Raul, he has more arms than a giant squid. Seriously, some like the convenience of having more then one cartridge always available; big mono collection, just like to have different presentations of the music, whatever. My two phono stages can be connected to two turntables[or arms] at once so I can demo more than one table without long hook up period. I once used two arms but ended up using one all the time; but if you like playing with cartridges multiple arms is a great way to go. Yes, you could hear the differences. ALL turntables sound different and the more expensive ones are almost always better. Worth it? Your call.

just to show off

I would love to have a universal turntable with three arms - one for stereo LPs, one for mono LPs and one for 78s. Unfortunately, that would require too many compromises. But a two arm deck is fairly easy to do.

Is that showing off?

Why would a turntable need more than one arm? A turntable is an inanimate object that has no emotions whatever. It does not "need" anything. The owner of said turntable may have any one or two of several reasons for wanting to mount more than one tonearm. These are not hard to imagine.