[2022] Is there a lore reason for Senna's followers (aka Sentinels of *Light*) to have powers of darkness?

It's not hard to see why Senna can use Darkness, but why do some of her followers (specifically- Solari Sentinel, Ixtali Sentinel and Dess & Ada) also use it? Isn't Senna supposed to be a special case or something in that regard? Or is it all purely about gameplay?