I have a simple question. I own all the old games that I like on CDs. What could be the reason that I'd purchase games here?

1. being able to easily download them to any of your computers.
2. these games are pre-configured to work on newer systems, in case you werent able to make them work yourself.
3. extras.

Convenience, more-or-less guaranteed compatibility with Windows 7+ without major tweaking. Some bonuses.

No DRM. Don't have to have the CD in the drive when you play.

Edit: Supporting GOG so that they can release old games in the future you don't already have.

1. It's much faster and easier to install from CD on any computer than to spend traffic and wait.
2. Just re-installed all BGs, IWDs and PST on Windows 7 64 bit without a problem. Also, the enhanced screen resolution mod seems work fine.
3. Bonuses and extras for old games is a really funny reason to spend $10. They have been collected for years.

Any other reasons, please?

Instead of "No DRM" there have been "no-CD patches".

CDs don't last forever you know.

Edit: No CD patches sometimes remove movies and other things in the games.

There are no other reasons. In that case, GOG is simply not for you. It happens.

(That being said, most people, myself included, honestly don't feel messing around with CDs is better, and there are some games that are very tricky to get to run on modern OSes. Patching up Thief isn't much fun, and the time saved there might be worth the few bucks.)

I don't know about anyone else but I don't rebuy games I already own on disc, I just purchase the games here that I don't have or lost a while back. So, if you have some games already, don't buy them again. But, GOG has a large catalog, surely you don't alraedy have everything they offer?

If you support GoG, then the future of gaming in general will be better, because DRM will not be ubiqutous(i misspelled, i know). DRM is a threat not only to our gaming, but to our very freedom, because with DRM they track everything you do. So if you support liberty, GoG is for you.

Same here.

I've gone through and made my own installers for several of my CDs so that I have them updated to the latest version and patched to eliminate the need for the CD if necessary and possible. I hit the ones that had 16-bit installers first, since they required using an XP VM to figure out what the old installer does (I run x64, so the 16-bit installer won't run).

I use GOG to either discover great games I've never heard of before or to finally play great games I knew about but never had the chance to play.

Actually using No-CD patches is similar to pirating games...and that is something bad :)

The reason?
To show the Publishers that with DRM shits they screw the customers not the pirates :)

You only need to download it once. Keep a local copy of the installer on your harddrive / external harddrive / pendrive / similar that you copy over when switching computers / re-installing. And I'd argue it's a lot more hassle free installing from GOG with large games that came on several CDs. You just fire up the installer and let it run in the background. No need to swap CDs or watch the install.

If you already paid for the game... then not really. You're just applying an unauthorized modification to your game. :p

Because you get the extra convenience for next to nothing.