Nobody has the same definition of manager. Translation is manager, do not be confused. The theme is a deep pool ... but here you can start.
There are many types, in general they are talent managers. That's the label I prefer. Managers of the market possibilities of a creative product.
All of them fit there. Musicians, YouTubers, Intagramers, Actors, composers, writers, artists, and so on.
Each one brings his contribution - costume party.
Managers solve the artist's mess Multitasker. No one needs to know how everything works. Although as the owner of your project you must know how to identify which piece of the puzzle you need.
Or intuit it at least.
If you know what a manager, You do not need one. Simple.
El manager Must be an administrator who knows how to guide the artist through the entanglements of a market. The artist puts the content, that will never change, that should not change. He manager Must give the guidelines and puts into action plans for that content to generate money and to know the artist in the market of his choice. In the proportion they expect.
He is a guide, an adviser, a counselor.