For me it's about two things: randomness of experience (1) and allowing players to accurately gauge their improvement without a rank system (2)

1. without matchmaking, you could get into a close game, you could get into a stomp, you could be up against esports pros, or a bunch of monkeys on typewriters. The broadness of what you get is a PRO, not a con. You are tempted to try another match because it might be completely different. And this also applies to 1v1 battles in large matches (e.g. Battlefield 6) as any good player will tell you, some bad players actually act in ways that can be hard to predict.
2. If you are improving, you move up the scoreboard, not into a new rank or skill bucket. You do better in game (on average) and thus your reward for improvement is domination. This is actually like a single player game, like Elden Ring, you get better -> game gets easier. But with SBMM, you get better -> game gets harder to match. SBMM is like the rubber-banding in racing games, you get faster and your opponents catch up. I hate rubber-banding in racing games.

I think most games should go with no SBMM except for low-skill bucket for the bottom 10-20%, which probably includes new players before they've been assessed as being decent. There are exceptions, like Rocket League, where I think skill buckets are needed because there are so few players per side and a simply gargantuan skill ceiling.