What is the need of a hybrid in telephone networks?

Digital exchanges need receive and transmit signals upon separate two-wire circuits. It calls for two-wire to four-wire conversion. This conversion is normally needed for trunk transmissions in analog exchanges. The circuit which performs 2- wire to 4-wire conversion is termed as Hybrid. A transformer based hybrid circuit is demonstrated in figure. The critical function of a hybrid is to make sure that there is no coupling of signal from the input to the output in the 4-wire circuit.