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what is the purpose for holding the air or radio traffic during different situations

I would think it is to keep the channel open so who ever is in the situation could communicate to dispatch/others and for other units not to jam up the channel with their (possible non-emergency) chatter or cause the officer not to be able to communicate.

Exactly.
Also the tones they put on are to let officers know the channel is closed.
That also tells us scanner listeners its a good time to pay attention. ;)

Its so that they are able to get out, and know that the dispatcher will hear them and not get by non priority traffic... Its also to tell people that may be passively listening, like a sergeant that there is something worth listening to on the air...