If you start Portal 2 (if you have it) there's a "Robot Enrichment" option at the bottom that is a link to an in-game item store. The items allow you to customize bots for co-op with paint jobs, gestures, and since Valve loves hats you can also get hats.
You can also get other items when you earn achievements, but most of those seem locked to trading.
So if you buy items in the Portal 2 in-game store or if any of the achievement items are tradeable, you can trade them with other people for anything they might have in any of the Steam inventories (TF2, Steam, Portal 2, Spiral Knights, etc).
I'm fairly certain they added the Portal 2 tab to inventories over the 2012 Holiday Sale when they added the promotional items, and that everyone got this tab regardless of whether they owned the game or not. (Maybe to try to sell more Portal 2, but it's never clear with Valve.)