When departing a controlled aerodrome you should be including in your start call '..as per flight plan..', ATC Tower would then activate it for you.
If you are doing an uncontrolled aerodrome departure, then when making a airborne radio call with some controlling authority at first point of contact you would say something similar as above. -If you had not already made a phone call to ATC on the ground before take off.
By activating it your ETAs and progress etc are communicated down the line.
If you don't activate it you then rely on someone matching your reg on the radio call to the strip/computer info they have by chance, if at all, and you are not being 'flight followed' until this happens. You can argue that the final destination and enroute points would have your ETA as per the flight plan, but nothing to match up with you from in flight they are non the wiser.
As for Radar, someone has to initially tag you electronically on their screen with the associated flight plan for it then to stay with the target.
Happy to be corrected on the above.
Last edited by DeltaT; 4th Jul 2013 at 12:39.