What are the primary advantages of a mentoring relationship as part of a new employee’s orientation

What are the primary advantages of a mentoring relationship as part of a new employee’s orientation?  Use your current job or the last known job that you had in the past.  Was there a mentoring program?  If yes, did you have a mentor?  Was it mandatory or optional? If there was not a mentoring program, do you feel like you needed one? If you did not have a mentor, do you feel that you would have benefited from having one?