So why is this important?
It provides you with a mechanism to easily tag form items, typically text input fields, so you can add AJAX functionality to them.
In Carl's demo he is simply changing the the border color of datepicker items, as a proof of concept. This is exactly the same technique I use with ExtJS to:
- create AJAX lookup fields
- make textareas resizable (strictly speaking I select by itemtype)
It means that developers can easily hook in widgets and AJAX functionality, because the low level plumbing has been done for us.