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?
From an orthodox trinitarian perspective, Jesus talks to the Father at the Garden of Gethsemane which proves Jesus is distinct from the Father (multiple persons). God in classical theism is understood to be the only uncreated substance in reality. So if Jesus did not preexist the Incarnation, then He is not God. If Jesus did preexist the Incarnation in such a way that He always existed, then Jesus is uncreated and therefore God. There is only one substance which is uncreated, so there is only one God. But in this substance are multiple persons.

So to answer your question, the preexistence of Jesus is important to the trinitarian because it implies that Jesus is uncreated and therefore God. The preexistence of Jesus is dangerous to the unitarian because they are trying to assert that there are not multiple persons in the uncreated substance we call God, so they instead attempt to prove that Jesus is a created being.

It all comes down to whether or not Jesus is created or uncreated.

P.S. I noticed someone had downvoted you, and I don't know why they did that. I thought your question was well formulated and honestly asked.