Alt's why?

[Sicno]

In my case it's because sometimes I come up with different ideas for characters I want to play as, and that may involve a different race, hairstyle and name. Stuff that can't be switched on-the-fly via gear sets.

There's also the scenario where you have friends playing on multiple servers, or when you're curious about how things are on other servers.

[Veralidaine]

I level alts because for me it's easier and more entertaining to have the main story quests as a boost to exp than to just grind fates or leves. That way if I get bored with a character, instead of spending 10 dollars on a fantasia, I can just go play another and have fun.