The Declaration of Independence set forth a formal assessment of grievances against the British government and declared that, because their rights had been violated repeatedly, the 13 North American colonies were formally declaring their independence from British rule. It had the additional benefit of bringing all the colonies together as a unified group and provided justification for intervention by future allies, such as France.
While the British had established the American colonies primarily to expand the British economy, Britain had mostly ignored their governance, especially during the 18th century. When the British attempted to institute serious taxation in 1763, the colonists were shocked and protested. The primary grievance the Colonists complained of was the lack of a voice in Britain. They were used to being relatively independent and resented being treated like an imperial territory.
However, the colonies were at different stages of protestation. In Massachusetts, there had already been open rebellion against the British, while New York and New Jersey were largely populated by British loyalists. The Declaration of Independence helped convince the more reluctant colonies that now was the time to rise up against the British government. By Aug. 2, 1776, 55 men representing all 13 colonies had signed the document. One final man, Matthew Thornton of New Hampshire, signed it on Nov. 4, 1776.