Setting the needle valve rich allows for better lubrication of the glow engine, set too lean it will often quit from overheating thereby increasing additional wear to components.

The engine experiences ram air effect in flight into the air intake which has the effect of leaning the engine, a slightly rich setting is needed for that purpose as well.

The engine on its own draws fuel which is certainly assisted by the pressurized tank setup or perhaps a pump if needed provides positive fuel flow pressure in all attitudes of flight regime. Otherwise certain maneuvers ( high G negative or positive or perhaps inverted flight) would most likely case cause insufficient fuel flow through the spray bar to maintain combustion.