The Supreme Court opinion in DC v. Heller explains why that clause was included:
>The Second Amendment is naturally divided into two parts: its prefatory clause and its operative clause. The former does not limit the latter grammatically, but rather announces a purpose. The Amendment could be rephrased, “Because a well regulated Militia is necessary to the security of a free State, the right of the people to keep and bear Arms shall not be infringed.”
This is originalism as clearly supported by the numerous references to contemporary language, other writings of the authors, and comparison to other language in the Constitution. Have you read the opinion?
