That is, what do they do when on their own in space? What other types of duties do drones have. The Omega Directive told us that one thing they engage in is scientific research. I, Borg and Survival Instinct each had a small number of drones on scout ships that had crashed on uninhabited planets, but their missions were not made clear, though some sort of research seems likely. And although all drones are capable of assimilating, I don't think that's the prime duty for all drones. Some would be medical drones, repairing damaged drones and maintaining maturation chambers, some would maintain computer systems, though others would be primarily about assimilation. Though Star Trek makes it seem as if it's an everyday thing to run into other ships in space, it's probably more likely that a cube can go for months without encountering another ship and I'm sure they've got other things to keep them busy then.