How would the game change if True Damage ceased to exist?

Can somebody explain to me what the point of having a damage type that cannot be built against is? I have never really understood the point of true damage. A good player who is looking at his opponents items should be able to counter-build their opponent correctly but True Damage just negates that and makes it impossible to build anything against them. Why has this ever needed to be a thing and how would the game change if True Damage was eliminated from the game all together?

For some reason, its usually only AD champions that seem to have access to it the most, with exception of some mages like Vel'koz. This makes the already horrific AP/Armor itemisation that much harder.