Lets you look at the design process for a website.
Lets you see every single one of the design choices that a web developer made while creating a website. Something people who love attention to detail might like using.
Theoretically, should work for most (many) websites, (I tested it on Facebook and Stack Overflow), but as website size grows, so will the time taken in going through all the changes in such detail. Definitely needs more work. :D