Some games work, some don't. I bought homefront retail and activated it on steam.
This is a case where a store-bought game requires you to install Steam (if it's not already installed) and activate it through Steam. That's the DRM which to me is pretty unobtrusive unlike other DRMs out there.
There are other games out there that you can purchase from a store and requires you to get a Steam account, but not every game is that way.
For those games that do NOT require Steam, as another poster stated, you can't "add" it to your Steam library so that you can install it from Steam and download updates. You can create a short-cut to the game from within Steam to launch it, but it won't keep track of any achievements within Steam nor will Steam apply updates.