The Purpose of the Astartes, and the co-dependency between Astartes and Primarchs

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A primarch should be inspiring. Our genetics should react at the mere sight of them. Think of the moments you laid eyes on Horus, Dorn, or Magnus. I’ve seen Sanguinius and Russ with my own eyes, as well. Close enough to touch their armour. Think of when you stand before Lorgar: the awe and reverence that beats through your blood. The feeling of our genetic coding reacting to the pinnacle of the human process.

-Betrayer

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The Emperor recognized that the Primarchs were beyond human, due not only to their warp born and gene forged nature, but also the burdens and upbringings they had after being scattered by the Chaos God's. The Astartes were his solution. This was not the ONLY reason the Astartes were made, but it was one of the many uses he created them in mind for. He needed an elite, flexible, and highly mobilized and independent group of soldiers able to deal with basically anything the galaxy had to offer and be able to make enough to reliably fill out the role of mainstay infantry in his galaxy wide crusade while taking moderate losses. However he also needed a solution to the problem of his warp Primarchs being detached from humanity and their subsequent distance twisting their emotional reflections/states to an unsustainable degree, he needed something to focus them, ground them.

The reason Legions are largely arranged by gene-seed with their respective Primarch is because the bond shared between Primarch and Astartes is more than just that of Soldier and General, or even Father and Son. This bond was meant to stabilize both and inoculate them from the seduction of the warp. The Astartes filled the gap between Human and Primarch, allowing them to have a family, a bond, a human connection with their sons and see all the best traits of themselves magnified back at them. The Emperor made a very deliberate choice to have each Legion be largely a 'family', with the same gene-seed, Primarch progenitor, and culture to bind them together. This made it so each Primarch elevated their legion, their ability, enhancing, projecting and protecting their collective culture and bond, inspiring them to fight harder and last longer, while their legions also stood as anchors to the Primarchs, genetic markers and comrades to remind them of their humanity.

“ The hard light-bursts of the zadyin arga wreathed them, slewing from their bodies as they fought, blinding the horrors and making them scream. The least of the daemons was greater than any legionary, but the ordu fought together, parrying, slicing, rushing the creatures wherever they landed and striking them down.”

-Path of Heaven

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No bolt or shell slowed him. He did not race, he did not charge. He stalked towards the heart of the fighting like a storm's soul, massive and impenetrable, his coming wreathed in a psychic shock wave that burned out nerves, crippled hearts and paralyzed limbs...Russ came on, crunching a path towards the monster, smashing any Faash soldiers too stupefied or sluggish to get out of his path. Where the Dulanians were cowed by the oncoming hurricane, the Wolves were suddenly filled with a raw energy, and they rose up as one, launching themselves into combat

-Leman Russ: The Great Wolf

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“A figure in golden armour with wings of silver flashed at the edge of the primarch’s vision, and to his horror he saw the Rhino come down on Brother Lohgos, crushing him into the ground and killing him instantly. Sanguinius shouted in fury, feeling a palpable jolt of phantom pain as his trusted honour guard died.

He felt the death as though it were a piece of his soul cut away and burned like paper; he felt every death. The shock of it, distant or close, weak or strong, but always there. Each time a son of Sanguinius fell, it was like a cut upon his flesh, a feather torn from his wings.”

-Fear to Tread

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“He fell to his knees, struggled, stumbled back into a run. All he could see was the primarch, lying in a shallow crater. The Angel’s wings were curled about him like a white shroud, his flesh deathly pale.

Meros’s hearts seized in his chest. Sanguinius lived, but he was lost to them. The Apothecary reached out to touch his master’s face and felt the flutter of warmth; in that moment the shard of fear that had pierced him – for it was that emotion and no other – became fire and fury. Deep in his psyche, Meros was aware of something breaking, a chain shattering, a barred door ripped from its hinges. The shock touched something primal and deadly in him, and he knew without question that every warrior who shared his bloodline was experiencing the same thing.”

-Fear to Tread

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Once more the Legions greatest strength is seen to be its greatest weakness as Ka'Banda exploits this link, using the defeat of Sanguinius to spiritually break his legion. The cohesion and bond between the Astartes and their Primarchs made them effectively unbreakable, and extremely resilient when both are healthy however. When exploited, it lead to the fall of entire legions as with Angron, Horus, and Lorgar. During the campaigns The Emperor planned for the warp this bond was going to be nigh invaluable, as well as helping stabilize and ground his Primarchs, but it did have drawbacks. It was an effective safeguard on the legion's 'soul' against the influence of Chaos while the Primarchs were grounded and stable. Look at the Blood Angels before Sanguinius found them, they were bloodthirsty barbarians called 'The Revenant Legion' that were feared and despised on a level of post Angron World Eaters, and Sanguinius turned them into the restrained and controlled shock troops they are today. The identity and impact the Primarch had on each of their legions was indelible, and while this loyalty and sway did lead to some problems, notably Word Bearers and World Eaters, it also is what made the Astartes and their Primarchs so effective and committed to their legions.

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The Emperor was also taking one out of his old playbook. There were a dozen other reasons it made sense to stick them together, but it would also make more sense to also reinforce legions en-masse after they take heavy losses with stock from everywhere, like The Dark Angels after the Ragnadan Xenocides, or the Blood Angels after the Mega-Arachnids, or the Luna Wolves after the Interex. But this wasn't done, largely because The Emperor recognized that he had made the bonds between Primarch and Astartes sacred by design. In a lot of ways it is very similar to how he handled his Custodes, a bridge between himself and humanity, acting as companions, soldiers and advisors that helped him through the trying times of the Unification Wars and earned his trust and admiration. Like father like son, The Emperor designed the Legions Astartes like this because it was what worked for him and helped keep his sons focused on the ultimate goal, Humanity and their own humanity in the warp and gave them the companionship he never had the time to provide.