User blog comment:PurpleWii/The Evolution of the Tank archetype./@comment-1330314-20160425182609/@comment-1330314-20160426020747

It's true that physical or magic damage will always be deadlier against squishies than tankier targets, since the latter will build resistances whereas the former will not, but percentage health is nonetheless better-suited for dealing less damage to squishies than a flat amount, which is why it's so common on tanks. For example is huge against anyone with a full tank build (you could be dealing as much as 400-500 physical or magic damage, sometimes even 600), but on an average squishy represents around 200-250 damage, which is about as much as the base damage on any ability. By contrast, if your ability were balanced around a flat 250 damage, for example, it would feel terrible to use against tanks and bruisers, and if it were balanced around dealing 400 damage, it would be way too strong against squishies.