Interviewing a grandparent will help you to understand your family history because your grandparent has, presumably, a much wider understanding of your family history than you do. Your grandparent is much older than you and has seen much more of family history than you have. Your grandparent remembers events that happened in family history and people who died before you were born. Your grandparent can look at family history from a much different perspective than yours. Because of these things, you can learn a great deal about your family history from interviewing a grandparent.
But all of this seems very obvious. I wonder if you might be asking why understanding your family history will do you any good as a social worker. This seems like a more plausible question to ask.
Understanding your family history can help you because it can help you to empathize more with the kinds of people that you might have to deal with as a social worker. For example, your grandparent might have been an immigrant or might have parents or grandparents who were. Understanding those people’s stories can help you to understand the lives of immigrant clients better. As another example, it might be that your grandparent knows of people in the family who had problems with domestic violence or with being abandoned by their spouses. Again, this can help you to understand the plight of your future clients more completely.
Interviewing a grandparent can give you more knowledge of your family history and that can help you be a more sympathetic social worker in the future.