What Paul is saying is that God has given each of us unique abilities for reciprocity - our own unique ways for extending help to others. Take a look around the people sitting in this sanctuary. Look at the number of people here who can give something unique and helpful and wonderful to you, and God has given each you an ability by which you can offer something unique and wonderful and helpful to these people here. He reminds us that we are like a body, and suggests by analogy that we are like ligaments. I find it notable that he uses the analogy of ligaments here.
Ligaments hold our bones together and keep our organs in place.
Reciprocity is a vital part of what holds us together. Paul is so right on this. I'll give you an example. Imagine a wonderful congregation in which a large number of people commit to contribute to the well-being of the group. They hold to reciprocity strongly.