This is officially known as Steamworks, which offers a host of features to developers and end-users as well. For developers, Valve does not charge for bandwidth use, patching, or activation of copies at physical retail or third-party digital distributors. For users, Steamworks allows the addition of in-game unlockable achievements, VAC anti-cheat security, Steam community integration, easy matchmaking, automatic uploading of game saves to a user-specific digital cloud, automatic patching and updating of games, and in-game voice chat integration. Most (but not all) games with integrated Steamworks may be uninstalled, redownloaded, reinstalled and reactivated a virtually unlimited amount of times provided they are added to a users personal Steam game library.
However, users may only add games to Steam with a legitimate, unbanned Steam account, and there are some games that only come with integrated Steamworks when purchased from Steam itself. Many of the games listed below will allow users to add the game to Steam from any retail outlet, but it would be best to double-check with another resource.
It's also worth noting that access to these games are dependent on the life of Steam and the Valve corporation. If either were to be compromised for any reason, access to the Steam game library may become compromised as well.