Enables you to easily borrow someone else's layout as 'inspiration' that you can then tweak. Don't discount the number of designers/developers that use another layout (or elements of it) as a base to work from - I've seen many do it.
Before there's a "Plagiarising!" response, also keep in mind that the final product in this case usually looks nothing like the original but that an original often serves as a more effective starting point than a blank canvas.
Certainly if someone was to 'rip' an existing website design wouldn't they just grab the HTML+CSS directly? Wirify is definitely not intended, nor useful, for that.
In some cases. In others, you might be starting with a simplified wireframe (and these remove identifying brands, colours, etc) or you would screen capture and use it as a layout base in PhotoShop.
While I am largely asking the same thing, the only bit of answer I could find came from the video: "As you can see, the wireframe does expose a lot of the structural aspects of the underlying page, which can be useful for analysis."
Why would I want to do this...?