What is the necessity of phase transfer of refrigerants in a refrigeration cycle. Liquid to gaseous back and forth in a cycle? Thanks in advance

The basic Refrigeration Cycle is of two stages namely Liquid and Gaseous form in principle.

But in Practice we get intermediate phase like partial gas in Liquid at the outlet of Expansion Device (Flash Gas) and some gas bubbles at the end of condensing phase which has to be Sub Cooled for better performance of the Refrigeration Cycle. In addition we also get apossibility of liquid at the entrance of suction to the Compressor.

To minimise these normally a Heat Exchanger is used

Phase change is where the most heat transfer energy resids.

In the condenser: high pressure/temperature superheated gas enters and rejects heat to the air/water once temperature reaches saturation the refrigerant changes phase. Latent heat transfer takes place and liquid is collected. Cp/heat capacity is higher in liquid than vapor.

In the evaporator it's the other way around: subcooled liquid refrigerant enters the evaporator and absorbs heat from your cooling media air/water. once temperature reaches saturation the liquid starts boiling to vapor and takes more heat (latent heat). Gas is sent to the compressor.

the necessity of phase transfer of refrigerant in the refrigeration cycle  evaporator device .

the main responsibility of evaporator convert the refrigerants from liquid to gas by take the heat from room  which we want to  Reduce temperature for it .....

note the heat transfer between air and evaporator by conduction method by fan in DX models or by water tube in water chiller