One of the main reasons why it is important to know your own blood type is in case you have an accident and you need a blood transfusion. In this case the blood you recieve must be compatible with your own because if it is not it will cause bood cells to clump inside your vains, and may cause death.
Another reason it is important to know your blood type is in case you want to give blood donations to someone in need. In this case again both types of blood must be compatible.
During pregnancy it is also important to know the mother's and baby's blood type, in order to prevent come conditions.
So as we can see it is highly important to know your blood type, specially in the scenario of an accident when there is not much time to act and you need a blood transfusion or are giving blood to another person.