The purpose is to create more interconnectivity & playability within the fanbase by effectively creating mini-mega servers within their architecture. I have plenty of friends on Crystal & Primal that I cannot interact with outside of alts due to the fact I can only bring my main to servers within aether with the world visit, which makes it hard to play with them on every content since unlocking things like Zadnor take a while and can be a chore to do more than once. With cross world data-center visit, I'd be able to bring my main to any of them and hang out with them in any content. it would also make it easier for players to find statics for raid content as they'd be able to raid on any datacenter without having to physically move there, etc.

Basically, a win all around for playing with friends.

As for actual server transfers, if you want to:

-Join a specific FC
-get Eternally Bonded to someone on a specific server
-get housing on a specific server

You still need to have your character on that specific server as your home world, which the $ cost server transfer will give, so it'll still have plenty of uses.