1. The structure provides automatic accountability
I have three mentors and I am fortunate for having each one of them. This always happens: A few days before my meeting with each of my mentors, I start attending to several of my pending matters. My mentors dont even have to ask me about things I promised to act on. I will take care of these things because I want to be ready with a good answer if they do ask me. The mentoring relationship in a way has built-in accountability.
Sometimes you may put off answering some questions just because you can. You will do this even typically for questions where you know the answers are not pretty. By postponing asking the hard questions you are not solving any problem, you are just avoiding the short-term pain. Your mentor may not be so nice to you. He or she has no problem asking those hard questions and actually prompting you to start doing something about those questions.
3. You can learn to reflect
A mentor does not have an alternate agenda except to help get the most out of you. So you never have to worry about any other side-effects as you discuss your life and work issues. That in itself will let you open up and reflect on things at a level that you have never seen before.
4. Discover the real problem and get help to solve it
Sometimes we keep messing with symptoms rather than attacking the real problems. I have found time and again that I discuss a particular problem with my mentor and actually we end up solving the real problem. Solving the real problem will in turn solve the symptomatic problems that you first set out to solve.
5. You may escape from short-term thinking
Being in the technology world, you have no option but to be current buzz-word compliant to ensure you are in the race. This means that you have to be running (hard) just to stay where you are. While this is great for short-term success, you cant ignore your long-term goals. Your mentor may help you balance the time you spend between short-term and long-term goals.