I've noticed many sites (including stackexchange) use the 'fixed width' or 'boxed' layout where 50% of my screen is white space, kind of like watching a standard definition TV show on an HD screen.
Historically I guess this was for mobile device compatibility, but with fluid layouts, that's better handled by removing or re-formatting elements of the page when needed.
What am I missing? Why have many apps (facebook, soundcloud, stripe, mixpanel, etc) decided to go with the boxed format? Is it just a better user experience?