You'll reinforce this friend's ideas of the direction their life should take. They'll feel as if you're their teacher and guide, inspiring and cheering them on. Something about being there for them on their journey will actually help you along yours, too.
You'll help each other out on a regular basis by giving each other advice -- and it'll be sound advice, because you'll understand each other so well that you'll really have a good idea of where your pal needs to go next. The level of respect between you will really help you listen to that good advice and take it when appropriate! You'll help each other find a positive way to express yourselves and focus your energies. If either of you is floundering in any way in your personal life, this friendship will help you calm down, gain focus and find a direction.