Already owned games in other platforms

i am just using the steam option "add a non steam game" or something, for them to show in my steam game list, its more onvenient for me

I did that for those games. No difference and it is a little more difficult to apply the necessary patches, but they were cheap and it was nice to put up a review on them.
But if you don't want to review them or own all your games at one service, there is no reason. Square Enix hardly needs the money, Pyro Studios is content making mobile games, and there are no achievements or cards.

Maybe. If there is a steam workshop option on the game, or a steam only version.

only if it's super cheap.

The only two reasons I could see if that are:
-The game offers something more on Steam (steam workshop, matchmaking and such)
-The other digital distribution site is giving you issues somehow (trouble downloading the games, other site is going down (which is not the case with GOG, but it is with Desura))

All depends on the title. .Resident Evil for example, I own every single incarnation of the game. uber fangirl

There have been a few instances with PC games that were never on steam andd then eventually show up after I purchase a retail box copy. The Obscure series [has achivements now, so a tad incentive on replaying] also those Disney games. I owned around 75% of all of thoosee and yet I still repurchased them (@= Oddly, a steam copy of a game I have does not work at all, yet the retail copy works. Blech

Opposite problem, I'm trying to have DRM-free games, so i buy them on gog if i have them on steam. Also I hope that one day i'll be able to merge gog and HumbleBundle drm-free libraries.