It all goes back to Timothy Gallwey. Before Gallwey, coaches were people who wore whistles around their necks.
Coaches helped people get better at physical tasks. That meant they mostly dealt with sweaty people, except for swim coaches who dealt mostly with chlorine-blind people.
That changed in 1974, the year Gallwey published The Inner Game of Tennis. He shifted the focus from what was happening on the outside, to what was happening in the mind of the tennis player.
The rest, as the saying goes, is history. Gallwey tells the story of the Inner Game on his web site. It’s compelling, but it won’t answer the question about whether you should hire a coach.
What’s important about the Gallwey story is that The Inner Game was the faint beginning of what is now a hot field: coaching. Now there are coaches for all kinds of things that don’t involve sweaty physical activities or even whistles.