Offline installer. So a game gets removed from your library for any reason. Now you get a new PC and can’t play the game anymore. At GoG you get an installer that doesn’t check servers and can work with no internet connection etc. So even if they were forced to remove a game from your library, you still have the installer and can install it whenever you want. So if you keep a hard drive of installers, you will forever own the game as long as you don’t lose that data.