There are several reasons that you should know if you have a mutation that could potentially cause harm to your health.
One reason obviously is to avoid medical complications in future. For example, there are several mutations that can cause blood clots or high cholesterol in your body, which could trigger heart attacks or strokes in older ages. Bodily functions become less active and organs become more rigid when you age, which could trigger medical complications that do not usually occur at younger ages. Carrying harmful mutations could worsen the situation and cause unexpected medical problems. Now If you know that you have a mutation, which could cause complications in future you could start taking precautionary measures to avoid such problems.
Presence of family cases of a particular genetic disease is a major reason to get checked for mutations causing that disease. If a family member (a blood relative) suffers from a genetic disorder then you should make sure that you and your family do not possess that mutation. These relationships could extend back to several generations since mutations in one copy of the gene may not cause the disease but are passed on to the next generations and could potentially be harmful if they pair up with another mutated copy.
Family planning is another major reason to know if you have any mutation that could potentially affect your children’s health. For this purpose only one parent needs to get tested, if positive (either carrier or affected) then the other parent should get tested too.