I'm mostly interested in what their purpose are in Fallout 4 since I seldom play Fallout 3 or New Vegas anymore, though it would be nice to finally understand how they worked for the previous fallouts.
I remember in Fallout 3 I had no idea what they were for. In Fallout 4 I see sometimes they activate an emergency frequency which I'm not sure if that's just for authenticity or if there is a purpose behind them.
Best Answer
According to the Fallout 3/NV wikia:
As for Fallout 4, it doesn't look like they serve any important purpose (see here and here)