"Plate was proof" (meaning impervious) against the vast majority of weapons encountered on the medieval battlefield. That does not, however, mean ALL weapons. It wasn't undefeatable, but it certainly wasn't crap either.
Not everyone had shiny suits of full plate, not even most. Perhaps five percent of the battlefield were armoured "cap-a-pie" (head to foot -
dé capite ad pedem), or in "three-quarter plate" -meaning full armor, excluding the greaves and sabatons. (common among those fighting on foot). The rest were unarmored entirely or wore whatever they could find -commonly helmets, breastplates (often without backplates) and gauntlets.
Swords were all but useless against plate. They
can not cut through it, and they can not pierce it effectively; that's why the fighting manuals of the time are quite specific about holding the sword like a prybar and stabbing at the gaps -under the arms, the groin, the throat, eyeslits, etc. The same for spears and polearms -other than the poleaxe/
bec de corbin. An axe might hope to get in a lucky blow that could shear through a helmet or breastplate, but it's more useful for battering the fellow inside, and maces do the same. Cloth-yard shafts from longbows are "generally accepted" to be ineffective against armor at all but close range, at which distance most archers will have already withdrawn or changed weapons.
We can demonstrate armor's effectiveness by using real weapons against real plate at full force and speed. There are actually people crazy enough to do this on a semi-regular basis.
It's hard to get them to agree to be shot at with longbows, which is one reason why the issue is still up in the air, but the matter of swords and cutting and thrusting weapons has been
firmly settled.
So that leaves heavy crossbows, poleaxes, warhammers, military picks as a distinct threat to armor. And in fact, crossbows aside, these were weapons most often carried by other armor-wearers.
These weren't uncommon weapons, but they were a minority of the weapons faced on the battlefield. So a man in plate wasn't quite invulnerable, but he
was proof against the
majority of the weapons he could expect to meet in battle. But then, only about five percent of the people you could expect to
meet in battle would be wearing a lot of it. I think an advantage like that would be well worth the expense, don't you?