Imagine for a moment we're playing D&D in a Bronze-Age setting. We have a lone warrior with a bronze short sword and bronze breast-plate. Against similarly armed warriors, I would consider most attacks to be at the chest (AC3,) although attacks aimed at the arms or legs might use a different AC.
But what if the warrior is attacked by a goblin wielding a bone club? The AC is still 3, but I would consider bone to be less durable than bronze, so I'd give the warrior a defensive advantage. Likewise, if the warrior attacks a dwarf wearing an iron breast-plate, I'd still consider the AC as 3, but give the dwarf a defensive advantage. Give the dwarf an iron ax, and that's an offensive advantage against the warrior as well.
Furthermore, when our warrior strikes iron armor with a bronze sword, I'd give the sword a 1 in 6 chance of being damaged. That counts as an offensive disadvantage, and needs to be repaired. I'd only have two levels of damage: "damaged" (or "battered" or "rusty" or other appropriate term) and "badly damaged". Beyond that, the weapon is useless.
Parrying a weapon (or having your weapon parried by another weapon) made of more durable material likewise runs the risk of damaging your weapon. Having your armor struck by a more durable weapon runs a 1 in 6 chance of damaging your armor in the same way. Damaged armor or weapons can be repaired for about 1/4th the cost of a new piece of equipment, or half the cost if it's badly damaged; destroyed armor and weapons need to be replaced.