Imagine if you took your average homeowner (who is not a carpenter or in the building trades), lets call her Barbara, sent her to furniture-making boot camp. At 'boot-camp' she learned how to use all the tools that one might use to make furniture. After graduation you ask Barbara to build you a chest of drawers. Keep in mind that Barbara probably has used, bought, and may have even assembled a [Scandinavian style] chest of drawers, but has never build any furniture from scratch before. At the same time Barbara is making her chest of drawers, you ask a master furniture craftsman to also make a chest of drawers. Which one of these chests of drawers do you think will you be more likely to sell at a garage sale (if you are lucky) vs. a furniture store?
(I'm assuming that you picked Barbara's for the 'lucky to sell at a garage sale' award, and the Craftsman's for the the store, since that's where I was trying to go with this story.)
It's obvious that in addition to knowing how to use tools, and knowing what a good outcome looks like, you have to actually have experience with the process of building furniture to get a quality outcome. What do you think the outcome would have been if the Craftsman would have worked with Barbara to create the furniture. I suspect the outcome would have been of a much higher quality.
How many people have you seen promoted to a management position, sent to a week-long seminar on management and left to fend for themselves for some 'probationary' period of time without any other guidance. They have the tools, they know what the outcome should look like, but they still don't perform to expectations. What if they had a mentor to guide them, wouldn't the outcome be better?