When you print both sides of the paper and then bind it or staple it, you
need extra space on the right side of the even pages and on the left side of
the odd pages for the binding. The gutter measurement adds that extra space
to the "inside" margins. You could add the amount to the inside margins
yourself; the gutter is just a convenience.
As you change the gutter measurement in the Page Setup dialog, watch the
preview picture to see what it does.
