Why is it important to mention if in my family there are members with any medical condition?

Genetic diseases are per definition diseases that are linked to your genetic constitution. You inherit your genetic material always from your parents and you will pass it on to your children. Thus, you will always share 50% of your DNA with your parents and children. There is a high likelihood that medical conditions of your close relatives are also present in your genes. Any medical information of your family members are therefore valuable hints also for the interpretation of your individual genome.