Learning to fight in armor is a process. Imagine trying to perform complex actions that will result in your life or death while inhibited in your movement, breathing, and sight. Throw in the possibility of multiple opponents, attacks from behind ,weapons raining down on you, (at minimum effecting your hearing and vision even if they do no other damage) and suddenly the idea of being a knight in shining armor takes on new meaning.
You can’t cut a man in armor, it is difficult to hurt him with blunt force trauma, but you can always throw him down.
Practicing your wrestling in armor is one of the best ways to ensure that you are prepared for combat in armor. Outside of the threat of a poleaxe, your opponent will have to close and look for weaknesses in your armor. This puts him in close enough to grapple. One of the great tournament fighters of the middle ages Jacques de Lalaing, was known to grab whatever he could reach, hold his opponent at bay and use his dagger to strike the holes in the armor.
Matt Galas has a wonderful article published here on the deeds of this knight.