Well, the radio operator may have more than wireless on different frequencies......a bit like what we have at one of the units I work at. So we should then have a rough idea which mic to pick up and speak in. It also means that if you do a radio check on, say, Seaton Radio on 123.0 and Blogsville Military answers you telling you are transmitting on 369.0 you might have enough symptoms to forward to your avionics engineer to get your radio fixed. At some aerodromes the poor bloke in the tower might have umpteen frequencies on speaker and if you don't say 123.0 he will put his tea down and be rushing up and down the line answering them all and end up transmitting over the top of your second call and nobody will be any the wiser and you won't go flying because you think your radio doesn't work when in fact it did and the bloke in the tower thinks 'I wonder who that was calling'. Et cetera. If you see what I mean.That's probably why......