That's for valve games and that 'valve pipe' stuff.
It's not just for Valve games, a dozen of other games use it, only outdated games use the username folder
And i don't know how Steam Pipe is bad, the only issues i've had with it is when games first had to transfer over to it, my games would still think they're in the username folder, but then that all got sorted out, now the only problem left over is an outdated Source SDK, so i'm forced to use Team Fortress 2 or Source Filmmaker for model compiling and viewing