You didn't read the text when Lumia explains?Honestly I don't even understand the appeal of soulbinding. You're literally cucked out of your soul in the lamest way possible.
BTW how exactly does it work again? Do they just say "you can hold onto it but it's still mine" or do they just yoink that shit? If the latter, what's the excuse on not becoming a demon?
She and Nareva have gotten really good at creating good imitations of a soul, which they fill their Valkyries and Astrida manders with respectively.
The idea of a soulbinding is a transformation that changes your characters, locking most of the TFs, unlocking unique powers and changing how the NPCs of that faction interact with you.