I didn't dig too hard for sources because this is probably a very minor expansion on top of the other answers, but WHICH payloads might require vertical integration? Ones with big stinking mirrors inside like spy satellites and space telescopes.
This reddit post lays out some rationale:
This is mostly telescopes like Hubble. They have a very delicate ...
For the shuttle program, the water was not recovered.
The water that was not vaporized ran through concrete channels and was collected in two holding ponds.
(NASA photo, annotations mine).
The water was later pumped out and allowed to soak into the sandy soil.
The discharge of deluge and firex water (during the launch of each
Space Shuttle) resulted ...