Friends serve a number of purposes. Some make us better people, some accept and support us for who we are. The best ones manage to do both at the same time.
But I think looking at a friendship from the angle of how it can benefit you and make you a better person is a somewhat selfish metric upon which to judge the quality or value of a friend.
I have certainly learned a lot from my closest friends over the years and I'm better for it, professionally and personally, but it's not the reason I chose those particular people as friends. It's a side-effect of having friends with similar interests and values, who happen to have different skill sets and different perspectives on life.
Neither is the sharing of that wisdom intentional, either, but rather it just sort of organically happens over time as we share experiences together and help each other out with work, family, and life in general. The motivation isn't to "make me better" it's to "be a better friend."
We are, after all, talking about friends here, not co-workers or colleagues or instructors or employers. This "bottom line" just feels rather shallow and selfish.