Why it exists
A founder ends up surrounded by people who depend on them. The management team has incentives, the shareholders have interests and the family is afraid. None of the three is a good sounding board for deciding whether to close a business line, let go of a trusted executive or turn down a funding round.
Mentoring fills that gap: someone with no agenda inside the company, who knows the operation in enough detail to have an informed view and does not depend on the decision you make.