Regardless of the size of your firm, every firm needs a managing partner.
If you’re a solo, there will be things that you do that are managing partner functions.
As a firm grows. it struggles to decide whether it can afford to have a partner work full-time (or even part-time) as a managing partner or whether to employ a non-lawyer CEO.
The managing partner role is one that is not taught at university. And where do you go with your career if you become a full-time managing partner for more than 2 years?
Patrick McKenna has produced an on-line book (a short 23 pages) on the first 100 days of a managing partner. It is worth reading even if you are a small firm.