Its not really a question of how much it does on its own, its how great the health advantage it gives, at least for laning.
Sure 80 is preciously little of 1000.
But, 80 bonus damage+80 healing ontop 500 base damage is good right?
Its effectively a health lead of 180 gained if putting it that way, ontop the damage you did post resistances.
If comparing to PTA, it does a bit of instant AD damage on activation which is basically rendered null by armor, its the 8-12% bonus damage taken that is of interest.
But you know.....on 1000 damage done that´s just 80 to 120 extra damage, its only when you add in multiple damage sources and a greater number that it exceeds conqueror in effectiveness. But for 1v1 or with less damage conqueror is immensely superior in terms of health advantages gained.
That and the bonus damage is still just more AD/Ap damage done which is based on the attackers dmg&penetration vs resistance anyway, if pen is low or nonexistent then Conqueror is without equal.
After all, most carries cant get good pen until 3-5 items, but conqueror rolls in right from the start, which gives it a incredible advantage in terms of being present, active and effective vs the pen that only comes along much later.
Riven, Jax and renekton would be crap against tanks without conqueror, they´d simply get stonewalled by the armor until they get a black cleaver which is a poor build path overall for damage.
Items that just add normal damage are meaningless when the opponent has a fat chunk of resistance+health, you need either sustain or a means to get past their resistances to deal with them.....usually both, which is what spawned current conqueror.