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.

Ham radio is opposite of most comercial thinking. On a local ham repeater
most hams want ot be able to hear all that is going on on the repeater so
that if anyone puts out a call he can be answered by anyone for a casual
chat. GMRS users are usually only interisted in the other parties they want
to talk to and not hear all the chatter on the frequency. That is why the
hook switch blocks out the gmrs and also releases the tone decoder when off
hook.