The Declaration of Independence has no legal standing today but is important as a foundational document that has helped create the American identity and mythology of a nation based on equality. The first paragraph is the one we usually read: the subsequent outlining of King George III's tyrannies are not of as much interest to us today.
The opening paragraph, which Jefferson borrowed and modified from John Locke, forcefully and stirringly expresses the United States's founding myth:
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
Most Americans know these words: they are part of the life's blood of how we define ourselves as a people. At the time that they were written, they expressed a radical new vision vis-a-vis European society (some scholars have argued that in addition to Locke, this vision was influenced by European-American's encounters with Native American cultures). At that time, European societies were organized on the principle of hierarchy, with a royal family on top, aristocrats below, and then the common people. Equality was not part of the picture.
We call the Declaration of Independence a part of the American mythology because equality has always been an ideal, not a reality. It is true that the US Constitution forbids the creation of an aristocracy, but at the time the declaration was written, men like Jefferson owned slaves, while women and Native Americans were treated inferiorly. The United States has struggled throughout its history to create true equality, but it is nevertheless important that the country holds that as a foundational and aspirational value.