Answer very briefly, the following:
Why is it necessary to know the blood group of the donor as well as the recipient?
Matching the blood groups of the donor and recipient is necessary during the blood transfusion to avoid incompatibility reactions.
If mismatched blood is transfused it leads to an antigen-antibody reaction in the recipient’s body. This can cause severe impairments in the recipient's body like clot formation. In some cases, it might lead to the death of the person.