Why do some FM transmitters have a power switch?

According to your question about the on/off switch for the Fm transmitter,
A long time before, there were such kinds of FM transmitters with on/off switches.
Because, that device also could select the channel (ຄວາມຖີ່), and this device type is connector by connector.
ຈາກນັ້ນ, even if select one channel (ຍົກ​ຕົວ​ຢ່າງ, 87.7), he could hear the sound of the external device (by using an FM transmitter).
ຢ່າງໃດກໍຕາມ, if he wants to hear OEM radio except for the 87.7 ຊ່ອງ, normally he could hear OEM radio channel 88.5 even if FM transmitter power is on.
But, at that time, even though he selected 87.7, another channel also could not hear, ດັ່ງນັ້ນ, it needs to power off about Fm transmitter.
After powering off the FM transmitter, he could hear the OEM radio from the OEM speaker.
That’s why needs to add a switch and needs to power on/off if the user wants to use OEM radio.
Our FM transmitter products do not need to add the switch.
Because our FM-16 FM transmitter is a wireless type, the user can adjust the frequency if he also could hear another radio channel during using your FM-16.