Chain armor was worn under plate armor and protected the body from attack.  The knight put the chain on first and the plate armor was worn over top as a second protection.  Chain linked armor especially protected from arrows because the head of the arrow could not make it's way through the linked circlets.  Plate mail was the most protective of the armor and covered almost the whole body.  They were suspected of being very heavy but they lightest plate armor, if it were smithed well, weighed only 45 pounds and still protected the body greatly.  A man would not be a knight without his armor.