A 30% cut for steam games sold on steam and a 0% cut for steam keys sold by the publisher wherever they want with the caveat that they must give steam users the same sales at around the same time. They get their games hosted on Steam's industry best CDN, a page with support for images and videos, an API with features users like, workshop API for mod hosting and delivery, and other SteamWorks API stuff for stuff like multiplayer, patch management without charging a fee for it, forum hosting to hit the highlights. Pretty much all of that drives engagement and is mostly turn-key though you do have to programmatically interact with their API when it makes sense.
Steam provides a lot of benefit for a 30% cut of what is sold on their store front and a lot more benefit for getting all of the above for a 0% cut if they sell steam keys outside of steam.