When the pads and rotor wear the piston extends out of its bore and can become cocked under hard braking. When the piston becomes cocked in its bore it can notch the bore and the cylinder rim can notch the piston. Once that has occurred the caliper is no longer safe as it will be apt to leak at the seal and be prone to seizing. And brakes is one area where it is better to be safe than sorry. Many years ago I rebuilt calipers and have dismantled seized calipers and the un-seized caliper from the other side and usually found similar damage to both sides. And if the stuck caliper is caused by a collapsed hose it is wise to replace them in pairs also.