Steam has a lot going for it:
1) Anti-censorship, stance of if it's legal it's on Steam. Plenty of filters for people who are easily offended.
2) Excellent refund policy that feels clear and automated and doesn't rely on the whim of a customer service drone.
3) Good backbone of Steamworks, community tools, groups, etc.
4) Has had better uptime/functionality over a long period than competitors like Uplay, MS Store, Origin, etc.
5) Excellent storefront where it's genuinely interesting to browse by genre, tag, upcoming releases, etc. With most other launchers I do my browsing elsewhere and then fire up the relevant launcher to buy the thing I've decided on. That's an important distinction: one is a true storefront, the other is a necessary speedbump to get our game.
Obviously more competition is not a bad thing (although it is a little ironic to see Epic imagining themselves as the new Steam after saying all kinds of unfriendly things about PC gaming in the 2005-10 period) but Steam is a genuine institution for a reason.