@Gyges Well, it is a pseudoscience so it's well-known for being bullshit, nothing new there. Still, like with zodiac signs and such you'll often find people believing in it on some level, it's pretty common afterall.
In mangas, authors usually do that to add more depth to the different characters in the story: you'll often see in extra/omake chapters some kind of "character info file" with their height/weight/birthday/zodiac sign/bloodtype/etc.