Steam's new disclaimer reminds everyone that you don't actually own your games, GOG moves in for the killshot: Its offline installers 'cannot be taken away from you'
Many of their games do have native linux versions, and a lot do work under wine or proton, which can be used as a Non-steam game in Steam or even without Steam.
Their launcher doesn't yet have a native linux version but it's completely optional, and does still run under wine if you really want it.
If I'm not going to use their game manager, then why would I buy the game from them instead of just buying it directly from the game studio? I guess because game studios rarely distribute their own games anymore?
Exactly, the game publishers and distributors are often not the developers themselves. Only one to distribute direct in recent memory was World Of Goo 2, and even that was sold primarily through the Epic store.
No clue if it's heroic exclusive but it's more than just affiliate linking. Heroic embeds the actual gog store page in the launcher and gets a percentage of anything you buy per their agreement with gog
Nowadays the Heroic Games Launcher is the preferred solution for downloading and running GOG games. It's a community-run project, but officially affiliated with GOG.