There are many functions that FBS supplies. Which ones are most important likely relies on the application. It provides attachment factors, growth factors, simple nutrients like amino acids and vitamins, stability factors for nutrients through binding, etc.The list is a very long one. These are the reasons it is difficult stop using it. I makes the culture of cells relatively simple and stable through providinges so many functions in one material.There is still more to learn about its functions in cell culture.