Even some of these features are transparent to users but not to devs, for example to say few:

• Multiplayer (big one)
• Microtransactions (eww!)
• Steam Audio (support for specific Audio stuff, not voice)
• Steam cloud (Save games)
• DRM
• Input
• Key management
• Playtest
• Steam Voice
• Valve Anti-Cheat
• Stats and achievements
• Workshop and not as important but relevant to some:
• Virtual Reality support

In a sense, this is not so different from what AWS is doing: Basically you offer a service with an API so your customers don't need to create the thing from scratch, but at the same time they become dependent of your exclusive services.