People are doing it for different reasons but my understanding was it was an opportunity to experience a whole expansion in a matter of a couple of months (rather than over a year and a half to two years), including leveling and getting a whole bunch of mogs and achievements it would take a lot longer to get (including some new ones).
And you can use all that as you will. Many are chain-leveling alts. I’m doing achieve and mogs and mounts and running my usual spec as a super explody-via-gems and speedy gnome rogue.