Rather than go into immense detail about why you would want to do this, let me present two real world examples of projects I have recently completed that have benefited from this kind of functionality:

1. An ecommerce site where the site owner wanted to arrange categories and products in a more logical order (e.g popularity) as opposed to alphabetically.
2. A real estate site where the site owner required the ability to upload multiple images of a property and then reorder them.

» Read more: Drag and Drop Reordering of Database fields with Scriptaculous.

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