Why are pets for? Do they really do anything? Do they evolve or something or are they just there to look cool?

They can do some minor things, depending on the equipment they're given. If memory serves, they can prevent your gear from being looked at by another player for instance. They don't evolve like Pokemon, but they do have alternate color pallets.

Pets have Three Rings that can be used

Drop Rate Increased for Quests by 30%
Instant Mailbox Access from anywhere.
Item Inspect Blocking

All these Require VP, aswell, you can get food for your pets from Pet vendors in every major city.

There's also:

Mailbox Access + TRANSFER.
Item Drop (which your pet randomly picks up items that drop on the ground, and they go to a small pet bag).
And also Bank Access, which I think I've only seen in KR.