An air conditioner compressor is the component in the system that raises the temperature and pressure of the vapor refrigerant that leaves the evaporator coil. Its important that the compressor raises the pressure of the vapor refrigerant so that it creates a pressure difference, the pressure difference is needed in order for the refrigerant to flow. High pressure fluids flow towards lower pressure fluid. Basically whats happening is the compressor is raising the pressure of the refrigerant so that the refrigerant will flow to the lower pressure refrigerant in the evaporator coil.
The compressor raising the pressure will also increase the temperature. The direction of heat transfer is from a higher temperature substance to a lower temperature substance, the lower temperature being in the evaporator coil and the hotter being in the compressor and condenser.
The temperature of the freon being increased is very important, because the refrigerant will get so hot that the hot air outside will be cooler even in the summer when it can be up to 120 degrees! The air being cooler outside allows the heat in the refrigerant to transfer to it when it goes through the condenser coils. The heat in the refrigerant was transferred originally from the heat inside the home at the evaporator coil. Its important that the compressor increase the temperature so that it can finish removing the heat from inside the home to the condenser.
Air conditioner and heat pump compressors are known as vapor compressors because they are not meant to have any liquid, and liquids are not a compressible fluid. Any liquid that may enter the compressor will result in reduced efficiency and capacity and will typically cause mechanical damage to the compressors internal components.