The Declaration of Independence had two main audiences. One was the American colonists. The other was people in Europe (particularly in England and France) who might be inclined to support the American cause.
The first audience was in the American colonies. While we might not realize this today, not all colonists were very interested in gaining their independence from Great Britain. It was important for the Patriots to gain as much support as possible from among the colonists. Therefore, the Declaration was meant in part to convince the undecided colonists that they ought to break from England.
However, it was clear to those who wanted independence that they could not do it alone. It was clear that they would have a better chance if there were people in Europe, particularly in England and France, who would support them. The English support would help to make England more likely to let the colonies go. French support might come in the form of arms, money, and even soldiers. For these reasons, the colonists wanted to explain to the English and French elites their reasons for trying to break from England.
The Declaration, then, was written with two audiences in mind.