I notice that in this question you ask why the Declaration “is” an important document. Therefore, I will address its importance today, not at the time when it was written. The Declaration of Independence is an important document to us today because it lays out the basic philosophy behind our system of government. It tells us what our core beliefs about government are as a nation.
The important part of the Declaration today is the second part. This is the part where Jefferson lays out his basic philosophy (borrowed from John Locke and others) with regard to government. Jefferson tells us that government is only legitimate if it is based on the consent of the governed. This emphasizes that we are a democratic society in which government only has power because we agree to give it power. He also says that we all have rights that are given to us by God and that government cannot take away. This is a very important idea because it tells us that we do not owe our rights to the government. It tells us that the government does not have the right or the power to take these rights away from us. Finally, Jefferson says that the only reason to have government is to protect our fundamental rights. Government is not there to enrich the rulers or to give them power. It is there to make sure that all our rights are honored and protected.
The Declaration of Independence is an important document today because it reminds us of these basic foundations of our system of government. It tells us who we are and what our core values are as a political entity.