|Hop on board the soul train.|
See, in order to deal with the Saint, Jesse gives him a tiny piece of his soul, which strips away his immunity to The Word.
In order to do that, the show needs to introduce the technological ability to take a soul, and in fact, take a fraction of a soul, and put it into someone else.
And in order to do THAT, the show has to give Jesse Custer a historical background in the supernatural black market, which they do by tying it to his family on his mother's side, the L'Angells, who, in the comics, were just some ultraviolent culty swamp hicks. I can't keep myself from thinking about how Jesse Custer knowing for decades that souls are a real, tangible thing because his family dealt in them messes with pretty much every Jesse Custer character beat so far. And it was all basically to service the plot, although apparently Jesse selling one percent of his soul is going to be a thing going forward.