It had several purposes.
1st of all, to declare independence from the Great Britain.
Secondly, as it stated:
..a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation.
In other words, to explain WHY they wanted independence.
Secondly, to inform as to the rational of why they had the RIGHT to do so.
We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable rights, that among these are life, liberty, and the pursuit of happiness. That, to secure these rights, governments are instituted among men, deriving their just powers from the consent of the governed. That, whenever any form of government becomes destructive of these ends, it is the right of the people to alter or to abolish it, and to institute new government, laying its foundation on such principles, and organizing its powers in such form, as to them shall seem most likely to effect their safety and happiness.
In other words, God has given every person liberty.
The Declaration went deeply into the entire Enlightenment's philosophy of God's direction for Mankind, and not just to separate from the British.
This. All 13 states were slave owning states. They were *NOT* trying to make a political statement on the issue of slavery. They were declaring their own independence was a human right, and they had no interest in side tracking their point by wading into the weeds of a completely separate issue.
This is exactly why the committee removed Jefferson's more clearly stated objections to slavery. It didn't help advance their cause, and it would have in fact blown apart the fledgling coalition they had created.
What was left was a Declaration that was slightly pro slavery, because one of the "causes" it listed was England's efforts to stoke slave rebellions.
"He has excited domestic insurrections amongst us..."
But the point remains, the purpose of the Declaration of Independence was to justify the Independence of the 13 colonies from a Union that they felt no longer served their best interest.
None of the signers(who represented their states) intended to send a message on the topic of slavery, though I say Thomas Jefferson should be given full credit for the manner in which his words greatly boosted the subsequent anti-slavery movement.
People saw it as hypocritical to demand freedom for themselves why they were keeping other people in bondage.