Same reason you play any MMO. Style, story, setting, characters, classes/jobs, progression, gear, all that stuff. All MMOs are pretty similar, as you'd expect from games of the same genre. If you like how it looks, plays and works then you should play it. If you don't, then don't. At a base level I guess it can be broken down into setting and gameplay. If you like the way those things look for this game, you should play it.