Shields and Helms

It is obvious how shields aided a knight in battle.  They were used for protection or to smash unlucky opponents close enough to be hit.  They were also ornamental and were displayed in castles as an heirloom or just for show.
The helm protected the head from blows from swords, arrows, and anything else hurled at them.  While they protected the head, they limited the breathing and vision of the knight.  Most helms made it very hard to see anything but a little in front.  The poor ventilation made the knight lose his breath soon after the battle started but were still important on the battlefield.