I will ask a question to the question: why are all people different? Each organism has a unique set of genes, why shouldn't it have different blood types? There are a huge number of other approaches to the systematization of blood groups (Duffy, Luteran, etc.), but the most popular and more significant is the division of blood into groups depending on the presence / absence of agglutinins and agglutinogens and by rhesus.
Blood groups are determined by the presence or absence of certain antigens and antibodies. Human blood plasma may contain agglutinins α and β (they are responsible for the "sticking" of cells when foreign bodies enter, therefore, when transfusing someone else's blood, it is imperative to take into account the blood group so that it does not roughly coagulate inside), in erythrocytes - agglutinogens A and B , moreover, of proteins A and α, one and only one is contained, the same is for proteins B and β. Accordingly, there are four valid combinations; which of them is characteristic of a given person determines his blood group:
α and β: the first (0) - there are no agglutinogens A and B.
A and β: the second ( A).
B and α: third (B).
A and B: fourth (AB) - no agglutinins α and β.
And the reason for the division of human blood into different groups is to perform a protective function against infectious diseases.