Why Ranks are Useful
A main reason we go players sort ourselves into ranks is so we can have interesting games with strangers. Folks who play each other often know their relative strength without a system. (A less "noble" reason for ranking is pride or vanity.) Without go's rank and handicap system, weaker players would be totally frustrated and stronger players would be totally bored. With the rank and handicap system, a 3-kyu player can say "My rank is 3 kyu," and his new 3-dan opponent will know what handicap to give for a mutually satisfying game. See steps between ranks.