How Water Towers Work | https://www.youtube.com/watch?v=yZwfcMSDBHs | by Practical Engineering

High buildings sometimes have their higher floors above the height level that the water pressure from the water supply of a town can reach.

Those buildings then use pumps in the cellar to either pump the water to a tank on the highest floor and let it fall down again to the floors or to pump the water to the floors directly.