RF modulators are useful devices for people who own televisions with RF/coaxial-only inputs because without the RF modulator there would be no way to connect a DVD player or video game console to the television. All RF modulators require an external power supply.
With rare exception, DVD players and video game consoles connect to televisions via S-video or composite video cables. They also require an additional audio connection, usually a RCA stereo cable. Obviously, these types of connections are not possible on a television that only supports a coaxial cable. When plugging these devices into a RF modulator, the end result is a conversion of the S-Video or composite and audio signal into one RF signal, which means your DVD player and video game console will now work on your RF-only television.
A video switcher is a great product for people who own a television that has only one composite video or S-Video input, but want to connect more than one device to it. Usually, people without a video switcher would have to disconnect one device and connect the other to get use of the device of choice.
But, those owning a video switcher could plug both devices into the video switcher, which would then plug into the television. The video switcher has buttons on the front that are married to the connection ports on the back. By plugging both devices into the video switcher, a person would only have to press a button to open the signal path for the device of choice.
This is a great tool for people who own a products like a DVD player, video game console and digital camcorder. Most video switchers are passive, which means they don't require external power.