It's not for medical reasons that they want to know. It's because, as stated in the first post, it is said to determine one's personality. From what I recall, A's are supposed to be the kind that are overly stressed out from being perfectionistic and driven, B's are supposed to be intellectual (I think, it's been a while), and O's can probably best be described by citing Eikichi Onizuka from GTO as an example.