A Bill. Further to promote the defense of the United States, and for other purposes. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That this Act may be cited as "An Act to Promote the Defense of the United States. (1)

This is just introducing the basic concept of the law and giving its official title. Phrases like, "be it enacted," or "Congress assembled," are old forms of syntax that are used in legal documents, partially because they allow more complex ideas and sentences, but also probably because they make the Senators and Representatives feel more important and fancy.

Whatever gets you through the day, right?