The Fisheye Loves Circles
A fisheye lens loves circles. A fisheye lens accentuates anything that is round. It makes circles seem more dynamic.
It is hard to explain this effect. I find the best way to explain it is to simply to show you. So here is a couple of examples of a fisheye lens making a circle more circular.
Yes you can take the same picture with a regular lens but they do not look as interesting as they do with a fisheye lens.
All I know is that a fisheye loves circles and it has this way of making anything circular more interesting in your photos.