Looks interesting. In the beginning I thought it would blur the part of the screenshot farthest away from you and keep the closest thing in focus. But that's not how it works.
I guess it could be useful for focusing on a particular part of the screenshot if you one could mark what the important part is.
Shallow depth of field is used in photography (and movies, sometimes) for exactly the purpose of focusing viewer attention on a specific element or elements.
EDIT: But now that I've looked at the demo...I am not sure what I would want this for.
I guess it could be useful for focusing on a particular part of the screenshot if you one could mark what the important part is.