BetterPiercing makes it so any damage dealt by an arrow with piercing ignores armor and protective enchantments.
Why you may ask? Because Crossbows are absolutely terrible against people with armor and they can't get the power enchantment.
Damage scales with the Piercing level, by 25%, so at level 1 it ignores 25% of the target's armor, and at level 4 it ignores 100% of the target's armor.
That's it, that's the mod.
