Families share common genes. environment, and life style etc. This means it is likely similar medical conditions might prevail in the entire family that were once associated to the grand parents or great great grand parents etc.
Medical History is important because of the following reasons:
1.Family history can assess the probability of risk that a particular member of the family might be suffering from a certain disease/disorder because his/her parents might have it.
2.Based on the past information, physicians can actually recommend solutions or safety measures to the family members so that something much dangerous can be prevented.