sunshinecorp: Yes, why would anyone buy ANYTHING on Origin.
Games that are exclusive to Origin? For what it's worth I've never had any issue with Origin after they changed their TOS/Privacy Policy, though I've not used it in a long time. It works about as well as Steam, as far as I can tell anyway.
Andremop: My answer is always: to support GOG and its mission.
That is always more important than minor advantages other sites have like friends list, streaming and even... GASP... price.
But that's just me.
pimpmonkey2382.313: This is the answer you should have been looking for.
Eh, for me Itch.io has moved past GOG in this sentiment, for me. It's far more dev friendly in nearly every respect, and it's just as customer friendly as GOG. And yes, it's DRM-free too. The only downside to Itch.io at the moment is that it's got a lot of "baby's first game" type stuff, but it's not exactly hard to wade through that.
Personally I don't much like the direction that GOG seems to be heading in. Things like hiding the GOG Downloader (which they'll be abandoning eventually), dropping the "One Fair Price" model, and restricting games, ignoring gaping problems (and/or taking several months to fix a glaring problem), etc etc are what have soured me. My guess is that as the GOG Galaxy moves towards a full release they'll find a way to do away with browser downloads.
Just my two cents.