My understanding is that water towers mostly exist as something to “pump against”, rather than being a vessel that gets filled and emptied repeatedly like a bat

My understanding is that water towers mostly exist as something to “pump against”, rather than being a vessel that gets filled and emptied repeatedly like a battery would. It does vary in level a bit, often with a circadian rhythm (but also randomly pulsatile). I just don’t think it’s a significant portion of the total water flow that its pressure supports.