It always puzzles me, why people need to store all the stuff offline
It always puzzles me why some are so overly puzzled by other people's motives. As if the people should not have the desires they have.
I can give you e.g. one example: someone has a shitty, or capped, internet connection at home, but a very speedy connection at work or school.
So let's say he's at home one Friday evening thinking: "Man, I'd really love to play some Far Cry 2 now!". So what can he do? Wait until Monday to get the game at work/school so that he can bring it home and install it? Or start downloading it through his shitty or capped connection at home? Yes many people have pretty flimsy connections, e.g. in many rural areas here the only broadband internet the ISPs offer are mobile 3G connections.
The option could have been that he has already downloaded all his GOG games to an external hard drive at work in one swoop, and just installs the game he desires from it. No need to wait till Monday for that.
Another reason to keep an offline library of your games is because GOG removed a Mac version of the game (which did happen, one GOG Mac user can tell you more), or the new 2.0 installer does not work as well with e.g. Linux/WINE as the old ones did. In those cases you might have hoped that you had the earlier version of the installer offline.
I'm sure I could come up with even more examples. Even though I have uncapped 10MBit/s ADSL at home, sometimes I feel I'd rather have all my GOG games on an external HDD, just in order to skip the downloading part when I want to install one of the games on a whim. For such a whim, waiting even a couple of hours may be too much.