Without going into too much detail, having HP allows us to create more varied units.
We can have units with low armour and low melee defence still be very tough to kill (e.g. Giants, Zombies).
We can have units with high armour be very weak if engaged by armour-piercing weapons (e.g. Irondrakes, Gyrocopters).
We can have units heal back up after taking damage (e.g. Trolls).
We can have damage that wounds all, but kills none (e.g. hot steam from a Steam-Tank's turret).
We can have damage that kills just a very few, but doesn't harm anyone else (e.g. a cannon shot).
And most importantly, we can scale up and down between all of these specialisations and characteristics.
As to why not all arrow volleys lead to certain kills, this depends on what you're firing at.
Mint-condition Ironbreakers? The arrows will just scratch the paint of their shields and armour.
Uncautious Goblin Spearmen hit in the flank from a decent height? Expect a couple of instant deaths with a generous increase after each volley.
There's a gameplay reason we're happy with the natural increase in kills - apart from it making sense in the real world too. If 100 archers were to take out 25 enemy archers per shot, the unit that fires first is certain to win the engagement. That means missile gameplay would be all about micro and less about tactics. It also results in more cautious advances and boring stand-offs at max range.
Hope this helps explain our reasoning
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KR, A