Do you frequently buy 30 games at once? Is having shitty forums/blogs inbuilt into a client a crucial feature... I thought we had the internet at large for that one but I guess walled gardens are fashionable? Similarly mod support is not a thing I would turn my nose up at but
https://www.moddb.com/ and a bunchof sites like it seem to still exist and being doing OK.
personally, when i get off my ass and finally buy new games, i buy 2-5 games at once. so a shopping cart is
mandatory, i very highly doubt doubt I'm alone
having a per-game forum is super useful for quick and basic access to support or a game's economy if it has one, I've poked around in the steam forums from time to time when i was too lazy to go to some other site to try to get other support, so it's pretty useful to have. the benefit to having a forum
dedicated to a single game outclasses various swiss army knife forums that do everything
edit: to add an extra point to this, a lot of the time you can get support from the devs themselves directly, or other experienced users, so you get a full spectrum of help. albeit the forums can be toxic or just filled with children that have no idea what they're talking about... but really what forum doesn't have that?
steam workshop is far easier to install mods than having to go to an external site like moddb or nexus, manually install the mod, or use their mod managers. mod selection is limited yes, but some people don't want to go super in-depth with modding a game
one thing steam has that literally no other storefront has that i enjoy, is uninstalling parts of a game, or removing DLC you don't use to save on disk space. for example, i can uninstall the campaign and singleplayer for black ops 3 and save myself somewhere around around 80GB. with black ops 4 on battle.net however, I'm stuck with the game being 150gb without being able to uninstall the multiplayer, battle royale, or DLC maps i don't play.