There is a thing on Steam, if a developer has a development version of a game and is not yet released on Steam, it helps, if people "wishlist" it there. That is why the developers advertise that Steamlink.
Also, there is shop integration. You can buy a game here on Itch and if everything is configured and the developer wishes to do so, the buyer will get a Steam key. This makes less sense for GoG, as games on GoG have no DRM, as most likely the Itch version also has not. Also, I presume the revenue share on GoG is worse than on Itch. While on Steam it might be similar, but you can't beat the reach Steam offers. I have seen games that are on Steam and on Itch and the Steam version has over 5k reviews, while the Itch version has 5.
The Steam platform does offer benefits when playing games with other people. There is no such thing as a friend list on Itch or capabilities to play a game together. Itch is a download store, not a platform to play games together with your friends.