My theory:
Men care about their facial hair. They artistically fashion their hair into all manner of styles- beards, moustaches, side burns, that silly looking triangle below the bottom lip... Girls hate any sort of face hair (face rash etc)
So why do men go to great lengths to style their facial hair like topiary? (Since only males understand and notice facial hair?)
To impress other guys.

Girls care about the way they dress. They take endless hours ensuring that their clothes are aligned and coordinated, that their handbag matches their shoes and that their jewelry compliments the outfit. Guys really don't care all that much- just as long as they look good. Not fashion magazine worthy, but decent-looking.
Why do girls bother? (Since only other girls notice the effort taken to coordinate an outfit?)
To impress other girls.

Not so different after all...