Many companies make the dangerous mistake of hiring someone simply on the basis that they have managed people before, taking for granted that they are an experienced manager who will not require any further help. In fact, nothing could be further from the truth. Managers are human beings too, and just as making home-cooked meals for a few years doesn't qualify someone to be a master chef, though it might be a good start, becoming a good manager consists of much more than having past experience managing some people for a while.
Management coaches enter at this point. Human resources are most beneficial to companies when they provide management coaching to help turn mediocre or poor managers into world-class leaders. Fortune 500 corporations will spend millions of dollars to train their employees with the best coaches the world has to offer. Bill Gates and Steve Jobs don't know everything either. They know this, which is why they're willing to spend so much money to train their people.
Take one example from the field of music: at a time when he was the most famous and well-paid living composer in the world, George Gershwin still took lessons in harmony from other composers! World leaders still take personal coaching in their field, which is a good indication that management coaching is an important part of bringing out the best in your management team.
Of course, one has to ask: where do you draw the line? Does everyone in a supervisory position need management coaching? Does a project leader? Lead engineer? Merely "senior" engineer, managing only themselves? The answer to each of these is a clear "yes."
Anyone making management decisions needs coaching, and the reason is that no one is perfect. We all had to learn things somewhere, but changes in the world (especially increases in business efficiency) require us to adapt and stay ahead of the curve. Like the kid's saying "you snooze, you lose", managers who receive no training "lose". They lose their edge, their team's advantage, and, if they are particularly bad managers, they might even lose their work force.
An angry lapse will never destroy a team, a bad day will never mean a bad month, and teams are led, not just managed, when they are the focus of competent management coaching.