I don't think there's much more to it. In any case, it's absolutely not needed. When you pay for the game, you will get access to all the downloads associated with it and you can download everything (the installer, often there are manuals, soundtracks and more goodies) without any limitations, and you don't need to jump any more hurdles to play.
(The sad consequence is that the GOG setup files are very popular among people who don't like to pay for their games — obviously there's little more a pirate could ask for than a complete game, often patched and set up to run correctly on modern systems, without any kind of protection. On the third hand, GOG seems to be prospering anyway, so apparently you can run a good business based on trusting people.)