It must have uses elsewhere but I can't figure it (auction sites maybe?)
Once loaded, just right click on the page where you want it. There'll be a new 'Reload Every' option.
This does carry a warning though. Make sure it's not enabled in the page that you're composing your post - cos every 30 secs it might just refresh and you'll lose everything!
* Actually, much more preferably - Get A Life!