Drawing on older lore here, the Blackstone Fortresses were built by the Eldar, who called them Talismans of Vaul. If the material that drives these weapons exists in another state and can be weaponized in various ways, then it stands to reason that whoever holds it would be a pretty big deal.
It seems pretty apparent what GW's play is here. Even with the return of Guilleman and no matter how well Imperial races play on the tabletop, from a lore perspective they are still very very much on the losing side of the survival game. The fact that the necrons want humanity alive may be their saving grace, as now there is a macguffin in play that could put Chaos back in their hole, and new individuals with the knowledge, means and potential to become an even bigger threat.
I must say, GW have done a rather impressive job of continuing the greater timeline of 40k. There is progress, regression, hope and hopelessness in all the right places.
Nonsense is our Salvation