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It wasn't so much the Primarchs but the Emperor. He stole a measure of their power to help boost His own abilities and aid in the creation of the Primarchs. He made a bargain then broke it so the Dark Gods messed with him.

Its not really the Primarchs themselves though, just what the Emperor would do with them. The Imperium still conquered the Solar System and thousands of other worlds without all the Primarchs but its obvious they just kind of speed up the process. Everything the Emperor did was to starve the Gods of power through worship and the Primarchs were just another tool in the belt, an important tool but a tool nonetheless.

The Emperor was going to do His big jaunt with or without the Primarchs though, He just saw a way to make it go a bit quicker. The whole thing with the Gods might have not even been an issue if the Emperor hadn't made his bargain in the first place but we'll never know.

The strategic goals of the exisiting Crusade don't really make much of a difference when the end game was always the same, unit humanity under the Imperium, purge the Xenos and usher humanity into an age of secular enlightenment until its realisation as a fully psychic species.
I'm not denying the Primarch Project was important because it was, just that if the Emperor had never gone through with the Primarchs then He would still have done the Great Crusade. The point being that the Dark Gods weren't afraid of the Primarchs, just the Emperor's plan for humanity. Everything else is just a means to an end for both the Emperor and the Pantheon.

The Astartes project was in conjunction with the Primarch project. The idea being the Primarchs would lead the Legion that was made from their genetics. Having the Primarchs present meant that there would be a purer source to create geneseed from, it's the Sangprimus Portum was important for the Primaris project since it contained untainted sources of the Primarchs DNA. Its why whenever a Primarch was found, the Legion numbers jumped considerably as they had a pure source of DNA to create more Astartes from.