It's the FCC's requirement. Emergency services radio must identify themselves on the air and one way of doing that is to have a computer generate the county call sign and transmit it by morse code. FM radio stations usually just identify near the top of the hour by saying the call sign "101.3, KDWB" but emergency services has to ID near any radio activity. That's why you hear it near a page or some radio traffic on the frequency.
I hope that makes some sense.
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