Where I've seen them provide huge value is in an organzation transitioning to agile, where the product manager/owner is more on the side of waterfall/sales, making unreasonable demands. The scrum master is the one who pushes back against this and "protects" the devs, and basically ensures that sprints run smoothly. Especially if you have thousands of developers transitioning to agile, hiring scrum masters that can each associate with 5-10 teams can make a huge difference.

On the other hand, if you have competent PM's (or even just eng team leads) who themselves serve as the buffer between the dev team and the rest of the org (sales, VP's, etc.) and who already know how to do agile, then yes, scrum master is entirely redundant.

But to be clear, leading agile isn't about reminding about Fibonacci numbers. In an inexperienced team, if one dev picks a different number, there's often peer pressure to just "change their number" instead of discussing the sources of disagreement, which is the actual purpose of planning poker. And there are a ton of situations like these where both devs and a PM might want to "play pretend" at agile rather than actually rigorously questioning requirements, negotiating better requirements with owners, and relentlessly surfacing blockers. So leading agile requires both real knowledge of the process as well as the courage to stand up against owners, groupthink and peer pressure.