A question about armour

I’ve been reading through the rule book and can’t seem to wrap my head around armour. From the book it reads as if it should be simple but the more I read into it the more I become unsure. Whats everyone else’s take on this?

Page 88 - Armor pieces are simple in that they have a protection value that adds boxes to your
stress track. (seems simple enough)

Page 83 - Armor piercing: The weapon can overcome armor effectively, and targets
do not benefit from any truths/benefits that come from armor.

None of the armour descriptions seems to list benefits or truths beyond the extra stress boxes that they grant so what’s the benefit of an AP round? Am I missing something or is this an error? The increased stress boxes is a one-off increase so I can’t just say that an AP round ignores that since it may take 2 or 3 shots against some tougher targets

If I am not wrong, in the latest update there is no weapon or adversary/monster that actually uses the Armor Piercing quality, so I believe the presence of the AP quality is something from an earlier version of the game that used a different armor system that they just forgot to take out of the book.
And it is not even the only part of the system that seems to contradict or make mechanics more confusing.