Roy Peter Clark has a post about using space on the page when writing, "giving equal weight to the markings and the emptiness." 

Use of formatting and space certainly makes a difference in readability.  My default left and right margins are 1 1/2 inches because I think the page just looks so much better that way.  The page seems more inviting, unhurried and gracious when the words are not crammed on every possible inch of page.  I get a little agitated and anxious when I read pages that are too crowded and I sure don't want my readers to feel that way.  Also, I can change the defaults and have some extra space if I am concerned about exceeding page limitations.