No offence taken.
Departure call gives traffic in the area a chance that may not have herd your other calls due to frequency limitations, their distance away from the aerodrome when you made the call, terrain etc, but importantly it gives a time of departure and more relevant traffic information to anyone who may be going to bump you at some stage.
IE if I hear a taxi call I normally don't pay too much attention, as they make take their sweet a&$@ time getting airborne or it may be one min, lining up, same thoughts also most wouldn't give intentions with a lining up call it would usually be ABC lining up RWY 01, which is good for inbound traffic not so much for others.... But, a departure call I know your at least airborne, passing x altitude and climbing to Xx tracking Xxx. You may not know I'm there but I can judge whether or not we are conflicting and either speak up or keep quiet and you would never be the wiser half the time.
As for the on the day bit...there are times when I've had a few aircraft trying to backtrack lineup and depart, before I've landed, it's one of many cases where I would give each leg until I knew no one else was going to enter, just giving the other guys enough info for them to use their judgement as to whether they had time or not to get out, if I call base, and they decide to backtrack, you could be sure ild report final, and most likely short final too if he is buggering about.