In Johnny Tremaine, what is the significance of Lieutenant Stranger's torn up letters?

The answer to this can be found two or three pages into
Chapter IX.  The significance of these letters is that they tell the Patriots when the
British are planning to move against them.

At this point in
the book, Johnny is working as something of a spy for the Patriots.  He has some
contacts who work among the British.  In this case, his contact is Lydia, the black
woman who does laundry at The Queen.  She has found letters that Lt. Stranger has been
writing to Lavinia.

In the letters, he has been trying to
tell Lavinia why he can't go to a dance with her.  One of his attempted letters shows
that it is because the British army will be marching out to the north on that
day.

So the letters show something about the British plans
and that is important to the Patriots.