Yup, our old motorola's had them aswell as our new airwaves. Once pressed, you get 10 seconds uninterrupted to repeatedly shout your location and give any important details about the situation.
But LOCATION is the key. Lost count of the number of times we get the panic alarm then get an officer telling us all the problems but not giving the location ... impossible for us to help and very worrying.
To be honest, more times than not it's a false alarm where officers have pressed it by accident.
You can see the "panic button" on the top of the GP340 below: (the speaker/microphones also have a panic button on them)

And below: pic of the panic button on a speaker/mic.