in boxing, martial arts wrestlng etc. it beomes quite important, heavier athleteic people tend be stronger than lighter athletes. weigh calsses put them on more even playing feild. ie if you were 5'7" 190lb, would you want to fight some one whos 6'9" 310lbs however some of the mixed matial arts, there are no weight classes, or they're spread further apart. you can get some iterestng fights, the smaller guys somtimes even win.