Yes, the sole purpose of bludgers is for them to be hit at players of opponent teams to knock them off their brooms. However, there are strict rules for who can hit bludgers at whom. Bludgers can be hit at anyone, although it is only useful when hit at the players of the opponent teams to distract them or knock them off their brooms. Bludgers must only be hit by beaters as that is their only function in the Quidditch match other than protecting their teammates from oncoming bludgers. Also, bludgers should not be touched with anything else except the beater's bat.