User blog comment:FairyRaven/Fairy Tail Chapter 456: Orders, Review and Commentary/@comment-6904925-20151010102931/@comment-1776824-20151010110337

Seilah's only use was a way to give Mirajane a power-up, since apparently she only has 2 other Satan Souls. Instead of giving her an original, new power of some unheard of demon, he opted for this: the cheap, unimaginative way out.