Reciprocity is a relatively new term in cultural anthropology, which refers to a procedure somewhat similar to barter. Still, it has an essential difference from barter, as barter is an immediate exchange, whereas reciprocity is an ‘exchange’ delayed in time. Therefore, interchange cannot be called a market exchange. The process can be compared to the birthday gifts exchange. Also, it is different from a simple gift, where no return is ever expected.
Why doing it? Psychologists say, it is an essential part of unconscious behavior, which cannot be rationalized. It is a need of our psychic to remain mentally healthy. Still, this is only one possible explanation. Giving, receiving and exchanging is a significant part of people’s culture, and it always was. So, we can also state it is a social construct to help people survive.
You can read more about it in works I find particularly useful: