First costume is +50% attack which is nice and good for regular titans if that is all you have for an attack booster (gives a bump to tile damage as you clearly understand).
Second Costume Grimm and Glenda add 40% damage to any special skill damage. That doesn’t SEEM like much and it isn’t for most heroes.
The niche is Pengi (and perhaps 9HB and the new dark elemental that hasn’t been released yet).
WHY?
Pengi can fire once per minion on the attacking team so up to 15 hits BUT the damage decreases on each subsequent hit. Adding 40% means that the last hits continue to do meaningful damage. This is huge on Mythic Titans where people fling mana pots and recharge for multiple firings in a row.
The thread that will give you the most information about the mechanics of how the special works is the Glenda Thread.
The Pengi thread has more information about Mythic Titans, Glenda and Mr Pengi and here is the Pengi/Mythic Titan Bug Thread that really goes into it.
Remember this is niche, and C2 Grimm is the economical way to get Glenda’s super power for Mythic Titans.