Users have downloaded games through the Games for Windows LIVE store will still have access to those titles, but will no longer be able to purchase new games after Thursday, August 22nd, 2013. Microsoft is planning to continue to support the Games for Windows LIVE platform and you can continue to enjoy the games that you have purchased, but purchase functionality in the client will be disabled.
The closing of the store has to with move by Microsoft to discontinue the use of the Microsoft Points system and transition to real local currency moving forward. It is also rumored that Microsoft is planning to overhaul its PC Game download service as part of this transition, but that has been yet to be officially confirmed.