It's mainly for role-playing purposes; My character is an acting taichou for squad nine because she's a good fit for the position
as a character in the realm of 'Bleach'. Biographies are for letting others know what your character does as a shinigami, as if they really are in the 'Bleach' 'verse; the more experienced shinigami are often ranked, and players that are more serious about RP give themselves ranks in a squad to make it more realistic.
I've quoted my short bio for reference:
Being from Rukongai 19, Makoto is full of potential and fighting spirit. Now, after a long and arduous process, she's somehow managed to procure a respectable ranking within Squad Nine, assisting Shuuhei Hisagi as acting Co-Taichou
