Fencing developed as a sport after Medieval Times. When gun powder was invented and enabled muskets to shoot balls through steel armor, soldiers stopped wearing steel armor. They still, however, carried swords for close-up battle. But now the swords became lighter in weight since no one was wearing steel armor and sticking a person with a sword, rather than slicing them with it was a bit more the style of battle.
If your group would like this knight to provide a brief demonstration of fencing techniques, this can be incorporated into his presentation.
For more information about fencing, see: