It gets the air traffiker's attention without clogging the airwaves and gives ATC the opportunity to continue when they are ready.
If you launch into lengthy waffle on your opening call you will often get a "G-ABCD standby" or "say again".
The controller may be on the landline, working other frequencies, expecting a readback from someone else, handling an emergency or just not holding their pencil. More time is wasted on busy frequencies by not doing this in my opinion.
On some handovers you are instructed to "Contact xyz, callsign only" because each pilot can barely get a word in. Someone who then comes on frequency with "xyz, G-ABCD is a Cessna 152 at, err, 2000ft on a QNH of 1013 with 2 POB, heading 180, out of EG?? to EG?? on a navex routing err Trumpton, Toxteth, Cockville, etc etc" causes mayhem, p1sses off ATC with irrelevent drivel and wastes RT airtime.
Last edited by Fg Off Max Stout; 11th March 2009 at 19:51.