The importance of mentors in any endeavor cannot be overstated. But l would clarify it little further. Crystallizing thoughts, learning new things goes through a definite process, requiring different expertise at its various stages. Setting out with coaching, subject matter experts, and then come mentor, guiding you all the way to initial success. Having said that these are role play and such an individual can play multiple roles, but surely are different in objectives and approach followed to accomplish that.