Having a downloader is a bit more convenient for getting game updates (you can always download the update manually and run it of course) and also for big games where you have to download multiple files to install. So it makes sense to want to have such a tool, as a big part of getting and retaining customers is convenience. But been there, I have done it, and it is doable, and sometimes preferable. Eg you may not want to install gog in a machine to play a game, or I may want to play a game through crossover but not download gog through crossover to get the windows version: with steam, I cannot do that. But even if you download the game through gog client for convenience, you do not need to run the gog client to launch the game anyway.