I would suggest that the inclusion of a list of grievances within the Declaration of Independence actually plays a critical role in justifying colonial independence. If you were to look past the Preamble, what the first part of the Declaration of Independence (beginning with "We hold these truths to be self-evident") aims to establish is a philosophical rationale for revolution (a rationale that borrows heavily from Locke in the process). Ultimately, this isn't so much a justification of the American Revolution in particular but instead represents a more foundational claim concerning the fundamental purpose of governments and the preconditions and factors that would make a revolt legitimate to begin with.

Once this rationale has been established, however, the colonials still have to make their case that the American Revolution is itself a legitimate political action along these terms. It is for this purpose that you see the list of grievances laid out, declaring the various abuses levied against the colonies. Ultimately, one could view this document through the lens of jurisprudence: first, it establishes the conditions that would define a rebellion legal and legitimate. Once it has done this, however, evidence must still be submitted if one were to make the claim that the American Revolution meets those preconditions. The two parts, then, ultimately work together to build this case to justify colonial independence.