[Formulas]

Atmospheric pressure is almost a constant, you need suction ( a delta ) or gravity for flow

Suction produces the delta in pressures to start fuel flow in a semi open system, high pressure seeks to fill low pressure void then after the pump fuel is pushed by positive displacement

Gravity feed is a bit more simple for the human mind to understand in terms of flow look at a water fall to understand that putting a fuel tank on a roof is an example of this

You need lower than atmoshereic pressure to work with the atmosphere in a semi open fuel system or gravity to drop it in