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 (frequency), and this device type is connector by connector.
Then, even if select one channel (for example, 87.7), he could hear the sound of the external device (by using an FM transmitter).
However, if he wants to hear OEM radio except for the 87.7 channel, 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, so, 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.