The reason we use fireworks as part of the 4th of July celebration is this:
John Adams (a and signer of the Declaration), wrote a letter to his wife, and said that Independence Day "will be the most memorable Epocha, in the History of America…It will be celebrated, by succeeding Generations, as the great anniversary Festival. It ought to be commemorated, as the by solemn Acts of Devotion to God Almighty. It ought to be solemnized with Pomp and Parade, with Shews [performances], Games, Sports, Guns, Bells, Bonfires and Illuminations [fireworks] from one End of this Continent to the other from this Time forward forever more."
And so that is what Americans do today.