Can someone explain to me how conquorer makes sense?

How does giving any champion in the game, the ability to do "true damage" make sense game design wise?

please explain to me the logic, of the mechanic, on how building an item, or taken a keystone, gives you the justification needed to negate any armor / mr in the game? What sense does it even make to pick champions specifically catered to that mechanic, if anyone can become a tank killer? Someone explain to me this design philosophy.... for the life me, I can't see it.

If anyone can get true damage, what purposes does armor / mr serve in the game?