User blog comment:Braumissomanly/Whats with all the percentage damage?/@comment-24464243-20140817200420

In some cases, I think it might not be for their effectiveness against tanks, but their effectiveness against non-tanks. Specifically, making them less effective against them.

Sure, 20% of your health as damage will kill you just as fast regardless of your tankiness, but 20% of a tank's health is a downright extreme nuke against a squishy target. So, if a champion is going to be going up against bruisers and tanks - like Nidalee, Gragas, and Gnar - then it makes more sense to give them percentage damage instead of flat damage, because if they had flat damage that was effective against their opponent then they'd dominate absolutely everyone. Their percentage components are a way to help prevent that.

Granted, in the case of Nidalee, she's going delete anyone she has the slightest edge of anyway, but that usually doesn't have anything to do with her percentage damage anyway.