Pretty much most has been mentioned by Rilla and Brovo. Showing the character's abilities and limitations up front accounts for fairness. Be it in an arena match, or a challenge a GM has tailored to fit the party. Having all skills and abilities as nice bullet-point lists are also a far easier reference than simply having them hidden in an IC-post. As a GM it is also a good way to gauge what kind of player you have on your hands, and very importantly if they are able to adapt if they got something wrong. Running systems may be a hassle to some, but tweaking characters together to create balance will teach you a lot about your players. 

Though one of the main uses of the bio-part of the CS is very simple; it teaches you if a player can read or just skips a lot of your OOC and fills in the blanks for their selves. This simple part has saved me so, so many headaches, you can't even begin to phantom.