A gas turbine needs high compression to force the air into the combustion chamber against the combustion pressure.
Well, not really. The combustor does not
create pressure, unlike a piston engine. In fact there's a small pressure
drop as air flows through the burner. The main restriction to flow is the turbine nozzle vanes (guide vanes) just downstream of the burner; this restriction, coupled with the pumping flow rate of the compressor, determines the pressure ratio of the machine.
It's very much like a garden hose; the spigot controls the flow
into the hose, but if the nozzle is wide open, there's not much pressure in the hose. But close down the nozzle, and the pressure in the hose increases.
