Because you can run the game anywhere, any time, without any extra launchers. You can install the game and play totally offline. You can install it on any number of PCs. If you have kids, you can install it on their PCs, without jumping through library sharing hoops and kicking each other out of the game. GOG themselves put a lot of work into modding / wrapping older games, to make sure they run in modern operating systems. They also have a 30-day refund policy (not based on playtime). They usually give away about a dozen free games a year. There are lots of reasons to be excited about a great AAA game releasing on GOG.