The term social science is a catch all for so many subjects it is sort of like asking "why should we study life"? Learning about our own identity, history, culture, and how we relate to other humans on this planet is essential to living as one. Psychology, Sociology, Anthropology, all of these are essentially the study of what it means to be human.
It is important to study social sciences because they are about what it means to be human.  When we better understand ourselves, we can better understand how the world works.  Once we understand that, we are able to identify root causes of society's problems and find more effective solutions.