layout - Is there a usability reason for centering a website's content area on a page?

A lot of websites these days have content area's which are center-aligned: http://ux.stackexchange.com is an example.

However some sites are left-aligned e.g. http://forums.adobe.com/index.jspa, http://www.area17.com/, http://30elm.com/, http://www.jefffinley.org/.

Is this just a design preference or is there a genuine usability reason for it?

Answer

I have yet to meet a client who, when presented with a centered layout, says "can I see it left-aligned?" so, of the theories offered up in the Centered Layout vs Left-Aligned Layout thread at the IXDA discussion board, this one seemed to make a sensible case for the improved readability of a centered layout:

Left-aligned may not be evil but on a wide monitor when one has a number of items open and the windows are not maximized, a centered-content design has more visual "white-space" around it and it makes for an easier reading experience. I've run into too many left-aligned sites which have poor padding allowances on the side margins which cause the eyes to work harder in trying to separate that site from the rest of the screen content.

--Ferg

Anecdotally: there's a moment of confusion when half (or, in the case of outlier QSXGA users, most) of a maximized window is painted with whitespace; plus the edges of the display tend to distract from the task of reading.

You might test this on yourself by dimming the lights and trying to read a 1000px left-aligned block of text at the edge of your monitor versus the same text centered. I tend to notice some refocusing/eye strain whenever the focal point of my vision rests at the edge of the display.