Why must air be compressed before fuel burns in a gas-turbine engine?

In a gas-turbine engine, air must be compressed before fuel is burned to increase the efficiency and power output of the Brayton cycle.12

By compressing the air, the engine increases the pressure of the working fluid before heat is added in the combustion chamber. Adding heat to the fluid at high pressure allows the subsequent expansion through the turbine to extract significantly more energy than would be possible at atmospheric pressure. This process ensures that after the turbine drives the compressor, there is sufficient surplus energy—manifested as high pressure, temperature, and velocity—to provide useful shaft work or propulsive thrust.21

Essentially, the compression stage is a thermodynamic requirement; the efficiency of the engine is directly linked to the peak pressure and temperature achieved during the cycle. Without this initial compression, the engine would lack the necessary energy density to operate effectively as a prime mover.2