well right now not very much.
you get a social profile page to post on walls and under the community dashboard. (community page in the menu)
however we're working on a stack database which will connect the accounts to the games library.
then that will allow you to submit media to any game in the database, add to your profiles and interact with other people doing the same. such as leave reviews, vote reviews, add your youtube or twitch content to the game etc and it will all become visible.
accounts right now are kinda lame. they just give you the ability to post or comment around the site. really, you don't even need one cause you can use your twitch account to connect to it. Oh and you can also get a store on there (like an individual marketplace)