As a PPL'er who is yet to pass my skill test, I have a thought or two on this.
Stay awake at the back please!
When I contact a Radar ATC or Information service I give them the information that PompeyPaul first mentions. I also give them an estimate to my next waypoint
relevant to them!
For example if I'm heading southeast through Southend RADAR's coverage, when I first 'pass my message' on joining the frequency I will give my estimate for overhead the Thames southbound because that's the normal point at which they ask us to report before changing frequency to Rochester INFORMATION.
In my humble opinion, the second example PompeyPaul gives is more relevant to those who have filed flightplans. In those situations there is no need to give as much information because that information is on the flightplan. The ATC
is interested in whether they are where they should be and 'on-time', hence the need to give a more precise 'time'.
The pilot who arrives unexpectedly on a frequency without a Flightplan will initially have to fill in the ATC's gaps of knowledge by giving the full flight information, but when he reports subsequent positions he'll be be able to give the same brief information as the chap with the flightplan.
Perhaps I'm wrong with my interpretation, but it seems to make sense.
