The purpose of the preamble is the outline the reasons behind the writing of the U.S. Constitution and the goals of the document. Although the preamble gives useful insight into the framers' intentions, the courts do not use it as a source of rights or powers that are not specifically enumerated in the body of the U.S. Constitution.
Constitutioncenter.org identifies the purpose of the U.S. Constitution as expressed in the preamble to be the creation of a just government, the establishment of peace and the founding of a healthy, free nation. The opening phrase, "We the people," emphasizes that the people and not the government are the sovereign source of political authority.
Wikipedia explains that the preamble is a source of much scholarly attention. The phrases "We the people of the United States" and "to form a more perfect union" have been interpreted as implying that the framers intended the U.S. Constitution as an agreement between the people and not between the states. Accordingly, it does not address actions like secession and state nullification of federal law.
There are notable exceptions to the tendency not to use the preamble in determining court cases. Wikipedia discusses how in the 1996 case of Ellis v. City of Grand Rapids the court decided in favor of eminent domain for the purpose of building a hospital by citing the preamble's express purpose to "promote the general welfare."