At first because they were cheap, which is important for a poor college student. I kept coming back because of the interesting games, Vista compatibility, and customer support.

Because I fucking feel like it?
I like games. GoG is cheap. Games that cost less means that I can buy more. Addiction follows, more games are bought than I can play at a time. Of course I also want to support the business model.
It seems that Ubisoft is starting to do something similar with some games (as seen with their newest Prince of Persia and HAWX, though the latter has a key required for multiplayer), which is a good thing. When I heard that Prince of Persia was going to be released without DRM, I decided to buy it just for that.

The compatibility is the main reason, vista can be a bit picky when its in a bad mood
The selection is the secondary reason, I get to find the games I heard about but never found or replace ones from other formats like Broken Sword
The price is a major factor what with my being poor (but still buying a game a week)
The age of the games are an asset since my laptop isn't the strongest machine around.
Also actually OWNING the games is nice

I buy games on GOG because I'm too lazy to search them on the abandonware sites :-P
Seriously speaking, I buy on GOG because I want to support the project and see how much far it can go. Anyway I'm still into the "package games" thing, so I consider games bought on GOG as some sort of "legit digital backup" of something I already own on disk or I would buy on disk in future....
Oh, and I'm also too lazy to set DOSBox to run with DOS games, so a nice icon on the desktop to let this thing run is a more than appreciated thing :-P

I also buy GOG games in he hopes that my meager addition of funds will bring some of the bigger hits here...
So basically, I am trying to level up GOG....*goes back to grinding*

*cue Final Fantasy style after-the-battle-music complete with a pan of the characters while they dance around a bit....*

I love buying from GOG because firstly, I live in Australia....not a lot of old games around here, so it gives me the chance to try those games I used to look EVERYWHERE for back in the day, legally and without DRM. Also, compatibility. Im one of the few Mac users here on GOG, so if I want to play an old DOS game that Ive always wanted to play I just put the files into DOSbox for mac and Im away!

Aww crud.... nearly out of gold - anyone have a rich countess spinster I can marry and kill off for funds? Must... buy... more....

1. NO DRM. I bought a lot of games on GoG that either didn't interested me at all of that I already own to show them my support for their DRM free policy. I am always one of the first to complains when a game uses online activation or against DRM used by digital distributor, so I found it normal to put my money where my mouth is when a DD decide to release all it's game DRM-free.
2. Lazyness - I own Fallout 1&2 since their release date but I have to admit that just having to download the GoG installer and play it 10 minutes later is a real pleasure compared of the hours normally spent on finding where I put the CD, downloading patch & crach, messing with DOSbox settings, etc...
3. Cheap - Some games that werent worth full price become sunddely a lot more interesting for 6 or 10 bucks. (Second Sight for example)

1. Selection of games
2. Compatibility (I hated messing with emulators)
3. Prices
4. Forums for each game (series)

In no particular order...
1. No DRM for games
2. Pricing is quite cheap
3. Good service
4. All games are supposed to be re-worked for modern OS's (Win XP, Vista)
5. I like EASILY finding old games on GOG that I just can't find in retail stores, online stores, etc.
6. No forced front-end program (i.e. Impulse and Steam)
7. Nice selection of games
8. I so happen to like supporting CD Projekt
9. The community here kicks butt.
10. The EXTRA Content rules -- especially when Soundtracks are involved.
11. Too many more reasons I can't seem to think of at the moment.

extra content, hard to find games.

No order...
1. No CD needed to play games
2. Compatibility
3. Convenience (already patched, don't have to dig through old CD's, install, patch, etc)
4. Fast downloads
5. no DRM
6. Well priced
- I recently purchased and downloaded 11 games from here, and 6 of them I already own in CD form, and an additional 2 are legally downloadable as free-ware (or a magazine subscription included the full version at some point), but I bought them anyway for the reasons listed above.

Because Vagabond held my sockmonkey hostage until I agreed to buy games from here. At least the money I save here makes poor sockmonkey's therapy sessions more affordable.