Flat-rate water bills remove the financial incentive to conserve, so people leave taps running and run half-full loads. Usage-based billing changes behavior. Simple pricing costs more in wasted water.
Chapters
If you know that your bill is not going to change, your water use is going to change, meaning you're not going to care whether you turn off the sink while you're brushing your teeth, or you do a load of laundry with 2 pieces of clothing in there, or run the dishwasher with 4 dishes in there. It's only when that bill fluctuates based upon the property's usage that does that actually modify the behavior of the resident. Doing it as a flat rate, although it sounds appealing as an owner because it sounds easy, You're actually not incentivizing your residents to conserve.