Armor proficiency points can also increase overall physical defense, though the benefits seem rather abysmal to me. More importantly, if you wish to equip higher-tier armor than the normal armor or high-tier shields, you need to invest in armor proficiency. As for Lyvia, I’m pretty sure she can wear normal armor from the start, even without any investment in armor proficiency. Since her default armor is a reinforced one, I assume she can also equip reinforced armor pieces without any proficiency investment, though I haven’t tested this.