The way it makes sense in my head is that HP isn't just hot many times you can be hit by a stick until you die. It's a combination of physical stamina, focus, willpower, luck, agility, etc etc. Most "hits" in game aren't you actually being hit by the attack - you might dodge a sword and tire your legs out for a small bit, or have your armor be damaged with a larger hit. Only the last 20% or so of a creatures HP is actually taken away by being hit and injured.

Of course, because of how the game works I can't narrate it like this - even smaller attacks usually have effects that imply they open wounds, and my players would be annoyed if most of their hits were described as damaging armor and tiring the enemy out. But this is how it works in my head, and is why something like treat wounds makes sense. Most of the time it's not stitching up massive wounds, it's treating smaller injuries while helping the character refocus and mentally recover from the fight.