ppdouble: and you dont hold all games installation backups, do you?
GR00T: If you don't, then there's not much point in GOG. Otherwise, may as well just stick to Steam.
While for me personally the ability to archive my games (so that they are accessible to me even after the demise of the store/service; see DotEmu for a recent example), there are other reasons too even when the store is still up and running.
For instance, someone here mentioned his woes of letting his kids play his Steam games, how the service boots one player out if the other one in the family tries to use the same account etc. There may be workarounds for that like always putting the other clients to offline mode and force them stay there etc. No such hurdles with single-player GOG games, you just install the games on different PCs and play them there, that's it.
Also if you have a slow and/or data capped internet connection at home but e.g. a speedy unlimited connection at work or the university or your friend's house, or even the local library, you might want to download your game to an USB flash stick in order to install it to your PC back at home, without having to install a client to the PC where you do the downloading. I've mentioned that e.g. the Steam client can't even connect to the Steam servers at my work place, apparently the relevant ports are blocked or something (plus one is not even supposed to install such clients to one's work PC).
Anyway, I personally keep all my GOG game installers backed up locally now so that if and when I just get an urge to try out some game, I just need to install it and run it, and not wait even hours for it first to download over my basic 10Mbps cable modem line. Someone with a 100Mbps or 1Gbps internet connection might care less of the waiting, I guess.