>I also assume that to ensure water pressure involves electricity - a lot of poorer countries do not have a constant or reliable electricity supply.

Today water pressure in the western world is mostly generated by running finely controlled electric pumps constantly, but it wasn't too long ago that water towers were used everywhere to maintain water pressure. The advantage of a water tower is that you can run an inaccurate oversized pump occasionally to fill up the reservoir on top of the tower, and gravity does the rest of the work.

Of course building a tower capable of holding many tons of water is a comparably complex civil engineering task and is costly in materials.