Digital Content Fulfillment is an underrated feature that ties directly to platform loyalty. Assuming this means the title is added to the Games Library list on humblebundle.com, customers love watching their collection grow and are more inclined to buy something if it will show on their preferred site (whether that's Steam, GOG, Humble, Epic Games, etc.).
Even scrolling through my GOG collection of games I'm somehow too lazy to play instills in me a sense of pride that at least they weren't purchased on Steam.
Though there are also major practical advantages of purchasing things within stores like GOG and Steam instead of a bunch of disparate websites: when you move to a new computer, you don't have to chase down a bunch of websites and license keys and emails to reinstall all of your software. Which is something I hate so much I only got outside of platform shops as a last resort.
