Conditions for water to go down from high places

• I’ve noticed that all of the above-ceiling water tanks are piped in a
  way that ensures to be open to the air at the tank, and then these water-carrying pipes go down to the kitchen or rest rooms,..etc

I mean that water won’t flow in the kitchen’s nor toilet’s pipes without being open to the atmosphere at the tank position.. Why is that?

I need a scientific explanation accounting for this phenomenon.. any general formula is welcome..