By allowing browsers to cache the image preferably: everybody gets a random photo, which then sticks for a while, but if you want a new one, you can always just force reload the page. But of course, it all stands and falls with the selection of images.

I have no idea wether visitors get anything out of this, probably not, but I use random color schemes on my personal site and it makes it much more enjoyable for me to work on it. Most people only ever see one page or two, but I have seen thousands if not tens of thousands, and a bit of variety can go a long way :)

Bing also shows different daily backgrounds.