How does Jesus' pre-incarnational existence relate to oneness pentecostalist godhead doctrine?

I've watched a few Youtube debates on trinitarianism vs unitarianism and the trinitarian apologists invariably try to prove Jesus "backwards" eternality (that he existed before the incarnation). But I don't get why this is so important? Why do trinitarians go after this argument so often, and why do oneness adherents resist it? That is, if it was hypothetically proved that Jesus was pre-existent, then what does that say about oneness godhead?