A Modulator is often used to convert the scart or phono output from a satellite receiver into a RF signal so it can be distributed to one or more TVs via a coaxal distribution system.

This allows channels to be viewed from the Satellite Receiver on more than one TV such as in the Bedroom or Kitchen

A modulator can convert signals from a satellite receiver, video camera, DVD player, or game console into a format that a television can use. It can also be used to distribute a single signal to multiple TVs.

There are different types of modulators, including RF modulators and digital TV modulators. RF modulators convert signals into an analog RF signal, while digital TV modulators convert signals into a broadcast RF television signal. 

Most HD Satellite receivers now do not have a modulator built in so an external modulator can be used to distribute the TV signal around the house.