This functionality is just for show now, but it might have some uses later
Valve is fervently working on the new operating system and users can now trick the Steam client to think that is running under SteamOS.Valve has started to integrate functionalities between Steam and SteamOS. In order to get a glimpse of that you will have to get the latest Beta.
All you have to do is to start Steam from the command line (because that's how Linux users roll) with the -steamos argument and check the system entry in Settings.
The operating system will read as SteamOS and that's about it. It also features an option probably aimed at game streaming, called “Enable Access to Linux desktop” that it's just for show.
This is the extent of the features so far, but it's a sure thing that Valve will not stop here and the future Beta updates will integrate more options.