The #page div is supposed to be centered on the page, however when you remove the “float: left” from the div, it no longer auto-stretches in height. If you view it in Firebug, you will see that without the “float: left”, the #page div ends behind the slideshow instead of extending down.
The problem is that a div does not automatically wrap around floated content. (By default, floated content hangs out of its container.) Using overflow: hidden is an easy way to force the container (in this case #page) to wrap around the contents.