Couple of reasons
1. One less number to keep in your head when giving (sometimes) long and complex radio calls.
2. "This time" means right now. If I were to say (at time 55) "Departed time 55" that means that everyone who has an interest in my transmission has to check their timepiece and do a mental conversion, only to realise "Oh, he meant right now!"
(I've never heard anyone giving "this time" as part of a taxi call)
Why is this a problem for you?