Blood types are to the Japanese what horoscopes are to Westerners, in that it's supposed to predict a person's personality. That's why a lot of character sheets will list blood type.
It's also why in lots of media (manga, anime, but others too), if there are four characters, you can be kinda sure that they will have all the blood types and they will roughly correlate with the abovementioned predicted personality. Notable example is Wotakoi. Very notable counter-example is Lucky Star, where they even point out that Kagami and Tsukasa have the same blood type (of course, they are twins), and their personality differs quite a lot.