The Declaration of Independence was written mainly as a way to get more people to favor the American colonists in their struggle with the British government. The Declaration did this in two main steps.
First, the Declaration laid out a philosophy of government. It talked about what makes governments legitimate and what those governments are supposed to do. Specifically, it said that governments have to govern by the consent of the people. In addition, it said that they have to protect the fundamental rights that people have, which are given in the Declaration as life, liberty, and the pursuit of happiness. When governments violate these rules, their people are justified in trying to overthrow them.
Second, the Declaration goes on to assert that the King of England had violated these rules. It argued that he and his governments had consistently done things to deprive the colonists of their rights. It argued that the people did not consent to be ruled by the British. Therefore, they had the right to rebel.
By saying these things, the Declaration was trying to persuade people of the rightness of the Patriots’ cause. It wanted to convince people in other countries to back the colonists. It also wanted to persuade more colonists to join in the rebellion. In other words, the Declaration is a political document that is meant to gain support for the colonists in their rebellion.
The title tells you why it was written..it is a declaration of independence. The colonials felt like they needed to do this to get badly needed help from France and possibly Spain. Both of those nations had monarchs and it would not have looked good to their subjects if they help overthrow a king....however, it the colonies were independent then they could come in of the side of an underdog...help defeat an arch enemy and possible gain favorable trade status to the west.