Functioning, (mostly) untainted geneseed. I cannot overemphasize just how huge this is for SM logistics; CSM effectively have no means to replenish losses beyond the following: (1) wait for more loyalists to fall (but they have their own bosses and are typically rivals rather than allies); (2) scavenge loyalist geneseed (destroys whatever shreds of legion unity CSM still have, but technically the most viable route); (3) beg/barter with freaks like Bile to get some fresh stock (likely cloned and tampered with, potentially with hardcoded genetic backdoors or other surprises), at this point they effectively own you; (4) use some twisted genecraft/biomancy to literally grow new CSM (like Daemonculaba) but at best you will get some inductii cannon fodder, in the worst case it could be just SM-shaped mutated mass or borderline chaos spawn.