No, it wasn’t really. They just wanted to conquer Earth easily, but now Anna is convinced that a) this “soul” thing is the source of human emotion, b) human emotion is an unpredictable force than can thwart their plans (and can also contaminate Vs in their human skins, c) this soul thing is a tangible thing that can be extracted and therefore eradicated from humans (and Vs), therefore d) finding a way to get rid of this soul thing will greatly help the invasion and should be attempted.
So The Human Soul is really the MacGuffin of the show and not important in itself. Won’t be the first time the show’s gone off-tangent, though, the goals of the Vs seem to change every month (first water, then “a mineral”, then visas, then killing humans, then breeding with them, etc.).