Planets deliberately salted with mined elements so that everything born there grows a harvestable system in its skull.
"They mine planets for the rare elements used to originally seed the worlds. These elements are inside of you."
Paulie's account makes pre-seeding the foundational act of the whole enterprise. Elements are stripped out of one set of planets and used to seed others; every living thing born on a seeded world develops a Natural Interface the size of a grain of sand in its brain, capable of interacting with the system. Earth is one of these worlds, which is why the crawl came to it.
The intended lifecycle was a closed loop — birth, life, death, and the element returning to the system charged with a lifetime of energy, sustaining a system that neither grows nor shrinks. Harvesting only becomes possible once the interfaces are switched on, which happens when a macro AI is installed. The Crawl is what turns a seeded world's entire population into a single shipment.
Ordinary worlds, indistinguishable from unseeded ones until a system switches on inside every skull.
The crop fields of the galaxy's economy — planets grown for what their dead contain.