Mirror margins allow you to 'mirror' the margins of one page onto another (Not the best explaination). For example, you print two pages and place them side by side. The left-margin of the left page will be the same as the right-margin on the right page (called the 'outside' margin), and similarly with the 'inside' margins.
I think this is more for when your printing on both sides of a piece of paper, ensuring that the margins are the same, no matter which way round the page is.
Hope that makes some sense.