Maybe I'm being confused, but what is the benefit of the upgrades that increase boss health/damage? They seem actively unhelpful to the player
We're updating the descriptions soon!
you increase your own stat(Damage/Health) with the boss currency.
New description will be something along these lines: "Absorb some of the King Slime's power to increase your Health"
Thanks for noting it, you're not the first to be confused!