Answer: In VCRS cycles, the heat is taken away from objects to be cooled by an evaporator.
The objects to be cooled are generally at atmospheric temperature.
[Remember, the heat transfer takes place from high temperature to low temperature.]
Therefore the temperature and pressure of refrigerant in the evaporator are reduced well below atmospheric pressure and temperature.
This process of decreasing pressure and temperature is done in the expansion valve. Where the refrigerator coming out of the condenser is throttled (isenthalpically). The throttling process makes the refrigerant reduce pressure and in turn reduce temperature.
In the evaporator, the low-pressure low-temperature refrigerant in liquid form changes phase to vapor refrigerant by absorbing heat from objects to be cooled.
Now this heat absorbed by the evaporator must be thrown away (rejected to atmosphere), hence the compressor takes this vapor refrigerant and compresses it to higher pressure and temperature and sends it to the condenser at atmospheric pressure(generally at atmospheric pressure and not always the case). The condenser rejects this heat in the atmosphere.
The compressor keeps the system working in the cyclic process continuously by compressing vapor at low pressure and temperature to high pressure and high temperature.