What it's useful for?
Basically any media gallery
But imagine you want to use avatars in your web applications: with this plugin you can avoid to develop a functionality that enable your user to load avatars of the right height and width, and still you'll be able to present them nicely in the size/sizes your layout need. Most of all you'll be able to change those sizes whenever you want without the need for your users to upload a new one, or even manage different sizes/ratio for different layouts with no drawbacks on the avatar chosen.
Take a look at the avatar demo
What this is not?
However with this plugin you'll just have to resize them to a more bandwidth friendly dimension, you won't need to crop/resize them to any particular aspect ratio, and most of all you'll be able to change that shape via css whenever you want afterwards.