Typically it's a temporary, an intermediate step, or politics.

I managed several teams, and during that time I inherited a new team with 2 headcount, plans to increase headcount, and company initiative to grow that product.

I didn't have bandwidth to manage that team myself, so I turned one of those headcounts into a manager position.

In the world of startups, the first hire is often an engineer, who normally reports to the tech-cofounder.

Say a small company employs a single senior subject matter expert who successfully manages up the chain to make sure the right work gets done. A junior subject matter expert will do better work if they are placed under the senior, because if they are a peer, they are likely to not manage up as well as the senior, who will have to step in and de facto manage to fix that anyway.

Not sure, plenty of individuals have coaches to keep them on track. I personally do better with accountability systems to keep me on track.

Yeah, I have no idea why a single person would ever need a manager. Just what exactly is the point?