If you like to do most of your developing in the browser using HTML, CSS you sometimes run into an issue where you need to test out some ideas in Photoshop. Somethings just aren’t possible with CSS and HTML. Some things are faster to use Photoshop in, but laying out boxes and content is faster is HTML.
Maybe you want to develop without relying on Photoshop so much. Well, it might not make sense, but you can then create your site in HTML and CSS first then have it converted to a PSD. Page Layers will break each element into a layer. This is also great if you need a PSD to try out some new ideas for a site you don’t have a mockup for. Or if your site has changed so much from the original PSD. And Fireworks opens PSD files so you aren’t just stuck with Photoshop. So yes there is a tool to go from HTML to PSD. Most tools go the other way around, but not this one.
Just to clarify there is a program called Layers which makes every item on your screen a layer in a PSD file. However, it only works with Safari right now and it will not break each element into separate layers, so its really just for taking a browser screenshot. Page Layers is the tool you want to use for HTML to PSD conversions. Unfortunately its only available for Mac.