What is the value of having a mentor as a young officer/enlisted?

The greatest value is the experience the mentor can bring to the table. They can take theory and translate that into the real world for the inexperienced.

For many, I think having a mentor early in one's career is possibly one of the most influential factors in being successful in the military (or any career). You'll naturally be assigned leaders who can fulfill part of the mentor role as your rater and senior rater. However, I'd recommend also looking for someone outside your chain of command that can help be a sounding board when you're dealing with something challenging at work or in your personal life and need someone to talk to... or to have someone give you a swift kick in the ass if you need it.