- My comment was becoming too long...
- The switch was not a on/off switch for the radio
- Poe had two radios in his craft
- The switch likely controls who Poe's microphone transmit to
So it is doubtful the switch was an on/off switch. Poe only has one headset and microphone from what we can tell to communicate with. This means the one setup must be able to handle all communications.
With Poe being a unit commander he needs to have at least two lines of communication. One with the command channel (to Leia in the command center to work the overall strategy) and one with his unit channel (to all the craft he commands on the battlefield).
With only one headset and microphone he needs to handle communication between the two channels easily - i.e. at the flip of a switch. Flipping the switch he can easily control who his microphone is currently sending communiques to.
It is also important to note that Poe would have to be able to hear transmissions from either channel regardless of what channel he switched his microphone to. Switching to one channel would be very dangerous if you could not hear immediate updates from the other channel that may take precedence or change the information that needs to be relayed. While the switch handled his microphone his headset still heard the audio from both channels.
While on the command channel any updates or Murphy's Law adjustments that need to be transmitted between the unit commander and command center can be sent and received to progress or adapt the overall strategy. The subordinates that Poe commands have no need to hear these updates or discussions, because they are supposed to be carrying out the previous orders they have been given and/or keeping watch for the bad guys. (Thinking about the big picture takes away your attention and could cause you to miss something...)
After sending and receiving the necessary information with command Poe the switches over to the unit channel to relay necessary information they need to know or any strategy changes that affect their unit.
You might also recognize that Leia can still hear everything that Poe said to his unit, because the switch was just for his microphone. It had not connection to any other ship or craft. In the command center Leia could hear everything Poe said, because she would need multiple radios that could both hear and communicate with any channel in her army... That said, Leia could have easily been able to tap into the channel and rescind Poe's orders...