part 2 THE BIG BANG
The main reason for the שמש to make announcements
is not to remind the Mispalelim to make the changes in
the Tefillah but it serves as a declaration (הכרזה) that
creates the חיוב of making the change in the Tefilah.
We find the concept of הכרזה in the Mishna
(Rosh Hashana 2:7), According to the תנא קמא,
Bais Din was required to announce "מקודש" even
when Rosh Chodesh is on the second day (which
happens automatically. if the new moon wasn't seen
on the first day) We still must declare מקודש to confirm
the day as Rosh Chodesh.
Similarly when Bentching גשם/טל we also must make
a הכרזה which declares officially the new season
for the Nusach in שמונה עשרה.
Perhaps the same תקנה applies when
announcing the start of saying טל ומטר / ותן ברכה
and יעלה ויבא we need to make a הכרזה to create
the חיוב of the new change in the שמונה עשרה.
Banging on the Bimah without announcing
anything will not suffice.