The bearing sits on the end of the clutch release arm, when you press the clutch the arm moves the bearing towards the clutch pressure plate and releases pressure on the friction plate. If the clutch has a few miles on it then as Serv said it makes sence to change the lot when the box is out. If you dont wanna fork out for a new paddle clutch and it's not showing any signs of wear (slipping etc) then just changing the bearing will get rid of the noise.