a. because they create social obligations to give, to receive and to repay (i.e., reciprocity), thus making people interact with one another
b. the circulation of presents binds social elites in different nations together, discouraging violence and conflict
c. the need to buy expensive gifts encourages people to work harder, financially stimulating the economy
d. when people see antiques, they remember a Golden Age in the past, and this reminds them to treat each other in a just and equitable way