There are two good reasons to understand genetic diseases, and to have some idea if there are any genetic diseases that may run on your family. First, knowing whether one carries a genetic disease may affect the process of planning to have children. For some couples, there is a high risk of having a child with a serious genetic disease such as Hemophilia, Tay-Sachs disease or Cystic fibrosis. Being aware of such a risk may affect the couple's plans on having children; a couple with a high risk of a serious disorder may choose to adopt, to use eggs or sperm donated by an individual with no history of the disorder, or to do more intense prenatal screening.
Second, some genetic diseases or tendencies toward genetic disease show up later in life. Examples include Familial hypercholesterolemia, certain types of breast and ovarian cancer, and several kinds of heart disease. In most cases, carrying the genes for these diseases is not a guarantee that one will get the disease, but the genes do increase the chance of the disease occurring. If you were aware that you had a genetic disease risk, you could be more proactive about prevention, screenings, and risk reduction, all of which could increase both your lifespan and life quality.