The Back Bone of the Movies Plot has been a Evolving one, the process which was...
*Ridley had a idea for how to expand upon Alien the Story about the Space Jockey but he and Cameron never got the chance to do this.
*Ridley had evolved his ideas and a meeting with Fox was made as Fox wanted to do another Alien Movie. And so Ridley and Fox had brainstormed a Plot.... and then started the process of getting Writers in to put these ideas to Paper....
*Spaights was brought in to look at Ridleys and Fox's ideas and then go away and make a Draft, which was then a case of back and forth between Ridley and Spaights evolving and changing things until we had the Final Alien Engineers Draft.
*AT THIS POINT... They had the Solid Foundations of where they was going.....
1) The Space Jockey was a Ancient Humanoid Alien Race, who had created and played a role in Evolution of Many Races in the Galaxy, Mankind Included.... we dont know the reasons for this as it was never revealed to the viewers via the Drafts.
2) At some point the Humans on Earth had started to rebel against their Gods (Engineers) who had evolved Mankind over Thousands of years, Technologically and Genetically. But Mankind had grown to be a Race who would start to not Worship the Engineers in the way intended and they also started to behave in ways that the Engineers Original Plans never intended.
3) At some point the Engineers had started to work on and conduct experiments to create and evolve Evil Biological Warfare that they had intended to us on Earth after a certain point in time 2000 years or so ago... we (Humanity) and gone to far, become too corrupted and there was no way back.
4) These Bio-Weapons however ultimately back fired and got out of Control and put a stop to the Engineers on LV-426 and the Space Jockey was one of those unfortunate Engineers who had suffered at the hands of his own Bio-Logical Warfare.
*Mainly Fox but a bit Ridley, felt that maybe Spaights idea was a bit to Xenomorph and they wanted to cover the Engineers a bit more and reasons why we was created and to be destroyed. But the basic IDEA is still the same.
*Prometheus was shot but then edited and things cut down as if to go for a change of plans as far as the whole Engineers, who are now not Gods but serve under a other Race/Power.
But these themes still seem to lie at the bottom of it all but its how they now choose to detail them.
*We was created for a Purpose, but was this against the knowledge or wishes of the Hierarchy.
*Was it with the consent/knowledge of the Hierarchy, but a faction of the Engineers chose to give us knowledge forbidden and evolve us in ways not intended and to worship them. Was the Hierarchy aware of this Act....
*Were the Engineers created for a Purpose, and they had rebelled against the Purpose and then Mankind was created by the Hierarchy to replace us.... The Engineers got jealous and set out to corrupt us to then also become something that displeased the Hierarchy.
SO TO ANSWER THE QUESTION...
Up until what ever ideas they had Prior to Alien Covenant, Mankind was Created for some purpose and most likely something to do with one of those above...
But then mankind had proved to be Wayward and Rebellious ourselves, the more Free-Will and Knowledge we was given led us to think more and only about ourselves and our own needs... We proved to be a Failed Experiment and needed to be Eradicated.
But the interesting thing is...
Was we to be Destroyed because of purely what we had done?
Or was we to be Destroyed to cover the Tracks of what the Engineers had done...... (creation or manipulation of Mankind against the will/wishes of the Hierarchy?)