darthspudius: Same reason all the lame games are important when they leave GOG?
But at least on GOG you don't have to care about the existence of the store and the service, when you finally get around to play it.
I am currently playing Dungeon Siege: Legends of Aranna (retail version), which I think I bought like 13 years ago or so. This is how I like it, my games being detached from DRM-shackles which could potentially prevent me from playing it years from now. (Sure there may be other problems preventing it as well like technical problems on future PCs, but that is beside the point; DRM is yet another additional and unneeded impediment on top of all the other impediments).
EDIT: And anyway, it is a Steam game, not a GOG game, and I don't think it is usually mentioned here if some game is going to disappear from some other service (like Steam, Origin or UPlay). So what makes this particular game so exceptional that everyone, also on other services other than Steam, need to be aware of it? Honest question, I have no idea what kind of game that is and whether it is good or bad.