What do you think the borg do with the colonies and cities they haul out of planets? Do they just convert all that matter to add to their cubes? Or are they just incinerated craters from torpedoes or whatever that they fire at the planets. (Yes I searched a few pages, this doesn't seem to have been asked...as a thread anyways.)
I'd wager it depends entirely on what was there before. If there's a chance for something new or useful to be added to the collective, it will be assimilated - depending on the current needs of the collective, something as simple as more drones might be a sufficient reason. If it's just the same old stuff they've encountered before and it puts up some resistance (futile as it may be)? Pewpew.
I always just assumed that they transmuted the matter to fit their needs and added the matter to the cube in question.
"It is though some great force just scooped all the machine elements off the face of the planet. " -Worf
Then why bother scooping up the colony at all? Why not just beam up everyone? And why DID the Borg destroy/assimilate the Jouret IV colony in BOBW in the first place?
It was a probing mission. They targeted a small Federation colony as an opportunity to learn something about Federation culture and technology without incurring any significant risk. Maybe simply ripping the colony out of the ground and leaving the scene minimized their risk profile and the likelihood of leaving any evidence behind from having drones on the ground for a period of time examining everything.
Plus there was the opportunity of getting classified StarFleet information. They could easily extract it from a recently assimilated Federation or StarFleet official living in the colony.
Q discribed the Borg as 'the ultimate users'. They were also shown to be very efficient. Buy using a tractor beam to pull the entire outpost into space, resistance would be very much futile. The Borg could simply pick and choose what they wished to assimilate or consume, and then simply discard the rest. Cold, calculated, and efficient.