Amazon thought it could compete with Steam because it was so much larger than Valve, but Prime Gaming's former VP admits that 'gamers already had the solution to their problems'
To be honest I really do prefer buying games on GOG. One day steam will go shit and we will be stuck with huge game libraries locked there. The day GOG goes dark I’ll still have all the offline installers of everything I bought.
There's always someone in the world archiving stuff, and with GOG the installers can be shared freely if they ever close shop, since they don't have DRM. With Steam that can be a lot harder, depending on the DRM they have
GOG Galaxy has the ability to download offline installers. They're listed under Extras on the game's page. It's arguably even better there than on the website because you can download those .bin files all in a single click.