I’m hard pressed to think of anything good Steam did for gaming.
It gives massive marketing to small developers that would otherwise be forgotten in obscurity. Those developers never could have afforded physical media to begin with. It's not like Walmart was ever going to stock a game like Sunless Sea or Knights of the Chalice or Slay the Spire or Terraria or Rimworld or... etc. The indie scene on steam has been killing it for the last decade or so, and it wouldn't have had a chance without it.
Steam has also been an excellent, stable platform for both discussion of games (both reviews and getting help, either gameplay wise or bugfixing/technical stuff) and modding. Yeah, forums existed before steam, but they were very niche and if signing up for steam is annoying, how do you feel about signing up for a different unique forum for every fucking game you want to even ask a question about?
As a platform, it's also attracted quite a few remakes/remasters/ports of old games that would have otherwise been lost to time. I'm not exactly thrilled with a lot of those, but it's better than nothing.
It's also made PC a more attractive platform. If Fromsoft didn't have Steam, there likely wouldn't have been any PC ports for any of their games, they'd all just be Sony exclusives and we'd be sitting around using emulators to play anything other than shitty diablo/doom clone #3314, which was pretty much the state of PC gaming before Steam came along, outside a handful of exceptions.
It's vastly improved multiplayer, making it much easier to organize and play games with friends. Sure, there were means before, but it's a hell of a lot easier to get a tech illiterate moron into a game by sending him a game invite on steam than sending him 3 pages of instructions on how to set up his firewall correctly.