Apoem question

the poem:

hats off!
along the street there comes
a blare of bugles, a ruffle of drums,
a flash of color beneath the sky:
hats off!
the flag is passing by!

blue and crimson and white it shines
over the steel-tipped, ordered lines.
hats off!
the colors before us fly;
but more than the flag is passing by.

sea-fights and land-fights, grim and great,
fought to make and to save the state:
weary marches and sinking ships;
cheers of victory on dying lips;

days of plenty and years of peace;
march of a strong land’s swift increase;
equal justice, right, and law,
stately honor and reverend awe;

sign of a nation, great and strong
to ward her people from foreign wrong:
pride and glory and honor, — all
live in the colors to stand or fall.

hats off!
along the street there comes
a blare of bugles, a ruffle of drums;
and loyal hearts are beating high:
hats off!
the flag is passing by!

the question: why is the opening line repeated throughout the poem