This question already has an answer here:
Presently, the two main content <DIV>s on a SO page,
#sidebar, have fixed widths hard coded into the CSS. This means that even if you make the browser very wide (say, streching over two monitors), the only parts of the page that get bigger are the empty white space on either edge. Meanwhile, if the page contains a code sample with long lines, it will have the most heinous sin of design: horizontal scrollbars.
This is even worse when trying to print, as the #sidebar is (properly) hidden, but its space is not reclaimed -- the right margin is just bigger, all while the code sample is now printed with the scroll bars.
A trivial change to the CSS, making the width of the #mainbar DIV a high percentage instead of a fixed amount, will allow us to have useful print-outs.
UPDATE: In response to the commentors who prefer narrow columns, my suggestion of completely compatible with that --- all you have to do is make your browser window narrower! (Windows are not required to be maximized)