It is so that you will hear the other users. That way you will know not to
try and talk while the frequency is in use by others that have a differant
tone setting.

If that's the case, then it's opposite in thinking to amateur radio, where a
repeater may have a PL decode tone, which will allow you to talk on the
repeater, but not hear anyone else on that same frequency. The local repeater
here is on 146.910 and another on 147.045. There's other repeaters that often
will start coming in on the same frequencies, but if you have decode on, you'll
never know you're transmitting while others are talking on the other repeaters.
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73! de Andy KC2SSB - WPYI880 (GMRS)
Beachwood, NJ USA! Grid FM29vw
http://vhfradiobuff.tripod.com