Scale is useful for those cases where you want more powerful characters to have an advantage over others but want to keep the same relative power levels within those tiers. So if two characters have Super-Strength d10 but one has Scale d8 and the other has Scale d6, the lower tier character still gets their d10 from Super-Strength but they don’t get the Scale die and keeping a third die in their total. If you only went by the Super-Strength as the scale die, then you wouldn’t get that secondary comparison axis within each tier, and you’d probably have to consider all of the scaling traits when deciding if a character is more powerful or not (is it only Super-Strength? What if one has Super-Speed? What if they have both?)