Though several games such as Dark Souls and free-to-play MMOs have left the Games for Windows Live framework it still provides access to certain titles that require it on the PC and provides access to a small social network with many of Xbox Live's own features. Best of all in the multiplayer games that do use it it provides some fairly robust matchmaking and game balance features.
What's in a name
Games for Windows Live provides chat via voice and text a list of those you've recently played with and other basic social networking functionality. Over time this has become harder to use except on games which still support it. Those games also benefit from GFWL's TrueSkill rating which helps ensure that players are matched with those of similar skill. Unfortunately this also results in it being harder to play with friends who will often differ in TrueSkill. Finally GFWL also provides access to achievements and a GamerScore which serves as a point total of the achievements you've earned.
Games for Windows Live has features are great for game balance and single-player fun but poor (and waning somewhat) for social networking. If you're fine with registering and keeping an account it is a great boon to those multiplayer games which use its matchmaking.