For printing out your document as double-sided, or if you plan on making
double-sided copies. If you didn't use mirror margins the inside on the
front page would have "x" margin and the outside of the back page would have
"x" margin. It would look sort of messed up not to mention that you might
not be able to see part of a page if you are binding or hole-punching the
document. This, in addition to using gutters, make for a good two-sided
bound document.