I used to wonder why too for a long time.
I think I got it figured out now.
It's really very simple.
When the score goes back and forth from 40-40 (deuce) to ad whoever, to
deuce, to ad whoever, to deuce, to ad whoever etc. and someone just
turns on the match or joins the crowd, they don't know how many deuces
there have been.
There's no way of knowing unless a friend/relative or announcer is
keeping track and tells you.
BUT.... because the umpire graciously calls 30-30 INSTEAD of deuce, you
know immediately where you stand in that game. You KNOW only FOUR points
could have been played in that game up until that point.
--
"I don't decide what's important, I merely reflect it." - Whisper