Water Pressure is a form of potential energy, like rolling a ball up a hill. The RO system is high pressure on the raw side and low pressure on the clean side - the difference in pressure is the consumed energy.

Right that makes sense, but if that’s a serious drain of energy then one would imagine clogged pipes cause dramatically more waste?

Seems like small potatoes compared to having clean drinking water.

Fair point. At utility scale the power requirement is non-trivial. Like anything, there's more than one way to do it, and there's pros and cons to everything. It's up to the owner to decide what they're comfortable with :)

I understand that the difference in pressure at the inlet and outlet of the RO system indicates that work was done moving the water through the system, but the RO system is there to fill a glass of water - it's not clear to me why the outlet pressure is relevant.

Are you saying that the amount of energy it takes to deliver a given volume of water, varies based on the resistance at the outlet?