It's an explantory/reasoning clause. I saw this somewhere and think it's a good example: If there were an amendment that said "A well educated electorate being necessary to the functioning of the democracy, the right of the people to keep and read books shall not be infringed", you'd have a very hard time convincing me (or I think most people) that said amendment limits the right to keep and read books only to those that are either well educated already, or by only the electorate (and not say, children or felons).

Likewise, if the first amendment said something along the lines of "A personal connection with his creator being necessary for a upstanding and moral citizen, Congress shall make no law respecting the establishment of a religion or prohibiting the free exercise thereof..." I think you'd also have a hard time arguing that athiests and agnostics don't have a right to free expression or to not have a religion forced upon them by the government.

Unfortunately the current definition of "well regulated militia" per the Supreme Court doesn't require either regulation (in the sense of laws, or even discipline and training) or a militia.