Let's get straight to the point. Effects that reduces damage by a set percentage gets better with more armor and magic resistance.

So abilities, like AlistarSquare Alistar's Unbreakable Will Unbreakable Will and KatarinaSquare Katarina's Shunpo Shunpo, with damage reduction, their damage reduction gets better with more armor and magic resistance. So in other words, it scale with those states.

For very quick overview of armor/magic resit, recall that for every 1 point of armor/magic resist increases your effective health by 1%. 1 point of armor/magic resist does not equal 1 less damage or 1% of reduce damage. For a more detail about armor/magic resist, check this.

With this basis knowledge, you can see that with more armor/magic resist, the amount damage you reduce begin to slow down with more armor/magic resist. So in turn, this mean, it takes more armor/magic resist to catch up to the percentage damage reduction.

Here's a table to show you some quick data. Let's assume a champion has a skill that reduces damage by 20% along side with armor. Total reduction refer to how much damage is reduced by after armor and damage reduction. At the end will show how much armor is needed to match it without the reduce damage from a skill.

Actual Armor Amount Total Reduction Total Armor Value Needed Reduction Armor Value
50 46.7% 87.5 37.5
75 454.3% 118.75 43.55
100 60% 150 50
125 64.4%% 181.25 56.25
150 68% 212.5 62.5

Well, with that data, you can clearly see that the 20% reduce damage is getting better with more armor. The same applies with magic resist. So a champion like Garen with Courage Courage scales really well with more tanky item and a item like Leviathan item Leviathan with full stacks is a lot better for tanky champions.

I hope this have been informative and helpful. Thoughts, opinions?