Good question.
Early on I was captivated by a chance to being able to finally play old games I love on modern platforms. GOG tech support seemed to know the onions, and tried to do anything they could, to help you to play games you bought. I know that, I spoke with several of them (not sure they still work, though). Sadly, sometimes even their expertise wasn't enough - they couldn't help me to make Rage of Mages working, and it is one of those games I couldn't finish in my younger days and had to return disk to a friend, who lend it to me in the first place.
Then GOG started to change, moving away from "good old games" paradigm, into "still old games, but more like any other store". It's not that bad per se, truth be told, I think their move toward Galaxy was far better than any other "steam-wannabe" online store. Honestly, my biggest gripe here is this Panzer Grau interface colour palette. Personal preferences only - I'm not getting younger, you know. And Wired (surprise-surprise) wrote a good article on how web became unreadable.
Regarding array of "more modern" games, GOG has no advantage over Steam, minus, maybe some "goodies" offered as freebies, in comparison with Steam, where you either have to buy those, or don't have access to them at all. So, unless I'm truly interested, I vote with my wallet - he, who offers better prices or offers game earlier or at all will have my money. I see no point picking one over another due to "loyalty" motifs or something.
Yes, if I really, I mean REALLY like certain game or studio, I'll probably buy everywhere to support developers, but that's pretty much it. Games-wise, there is no point coming here for other than "exclusive" old games that work (if they do). Certain games are not even offered here, probably due to infrastructure issues (offered by Steam) or, probably, to prevent playerbase fragmentation.