(On Windows) If you add a non-steam game to your library, you can have Steam still give achivements and track time in the game by adding a file with the game’s Steam App ID in a text file named “Steam_appid.txt” in the folder with the executable. The only drawback is that it breaks the overlay.

Another use of it is to play official multiplayer on pirated games that use Steam’s multiplayer services by using the AppID for Space War.