What is the use of modulator and demodulator in transponder?

Question

In types of microwave repeaters "regenerative transponder" is one type. In that we use modulator and demodulator.

Answer 1

I hope I understood the query -

Most communication satellites are radio relay stations in orbit, and carry dozens of transponders, each with a bandwidth of tens of megahertz. Most transponders operate on a bent pipe principle, sending back to earth of what goes into the conduit with only amplification and a shift from uplink to downlink frequency. However, some modern satellites use on-board processing, where the signal is demodulated, decoded, re-encoded and modulated aboard the satellite these type of transponder are regenerative transponder.

Here Modulator is required to transforms a digital stream into a radio signal on Intermediate frequency (IF) and Demodulator for the Intermediate frequency to the original one.