You don't really need to know you blood type since doctors can easily figure it out (although it would be easier if you did; plus you save money for a blood test). However, in Japan (and across Eastern Asia), asking someone about their blood type is about as normal as asking about your star sign. That's why some anime and game profiles (especially Soul games and Final Fantasy games) have the character's blood type on it. Not knowing your sign (to them) would be...weird.
Anyway, I am AB positive, which means I am "cool, controlled, rational, sociable, popular but also critical and indecisive." I think this fits me so, yeah, I like my blood type.
Also, since some people don't know, here's how the "donor/recipient" thing works when it comes to blood types:
A can only receive blood from A and O; can give to A or AB
B can only receive blood from B and O; can give to B or AB
AB can receive blood from ALL blood groups; can only give to AB
O can receive blood ONLY from O; can give to ALL blood groups
There is something else people need to know: it also matters if you are RhD positive or negative. People who are RhD positive do not have a reaction to RhD negative blood; therefore they receive blood from these individuals. However, a person who is RhD negative can only receive blood from another RhD negative person (although they can give blood to both). For example, AB+ individuals can receive blood from all blood groups, positive or negative, however, AB- people can only receive blood from RhD negative blood types (O-, A-, B- or AB-). An O- individual can ONLY receive blood from another O- person; they can't receive blood from an O+ person, although an O+ person can receive blood from both an O+ and O- person. This is the reason why O- people are "universal donors" and AB+ people are "universal receivers".