We take it for granted. How many of us have ever asked this question?
Although a concrete reason for the existence of ABO genes that determine blood types is not known, even after a hundred years of discovery, scientists believe that there must be an evolutionary advantage. Interestingly, in 1952 Bombay, doctors reported people that had none of the recognizable blood types normally found in humans. This phenomenon could mean that there is no evolutionary advantage after all, but these are very rare. Claims have been made that people from group A have the worst hangovers and group O have the best teeth, for example, but these associations are probably coincidental at best.
And interesting piece @ https://www.scienceabc.com/humans/humans-different-blood-types.html