Apparently, it fixes your particular problem
Seriously, it forces the browser to fully reload the webpage, cookies and all, when next after clearin' the website is visited. To oversimplify, a browser with a cached version of a webpage will pull up the cached version, rather than loadin' a fresh page. It will compare the cached version with the current version and update only components which have changed since the last time the page was visited. Sometimes, because of cached cookies or other files, the browser might "think" its displayin' correctly, when in fact it is missin' cookies, controlls, or other files needed to display the updated page - it says "Done" but displays either nothin' or a broken web page, or even reports it cannot find the page at all.
Again, there's more to it than that, but that's close enough to the concept for most folks.