The speaker of the poem is talking about America to Americans. Not that others can't enjoy or appreciate the poem, but the idea is to use the American past to give present and future Americans a sense of the determination and courage that are their culture. Longfellow was writing during a dark moment in American history, just before the Civil War, but the idea can be applied to any tough moment when Americans might need a heroic example to lift their spirits. If he had to fudge the facts a little and make Revere into more of a hero than he was that night, well, it's only poetry, right?
The poem, by focusing directly on an American audience, excludes the larger world from participating in its message of freedom and courage in the face of danger.
Longfellow picks the perfect event to highlight the danger and the possibility of the Revolution, using one person's actions to stand in for the beginning of a nation.