Difference here is that absolutely nothing is stopping you from still distributing your game that way. You are not forced into the Steam marketplace nor must you to be successful (ex. Starsector), it's simply generally good business to do so.

As a small game developer, the steamworks SDK takes care of so many things I no longer have to care about. I don't have to run NAT punching infrastructure, I don't have to run a presence server and matchmaking services, handle user PII, cloud saves, workshop mod hosting (which can get very large with texturing and media), etc etc. It has an attractive value as a game developer too, not just as an end-user.