As an app developer and designer, you always want to keep the big picture of what you're building in mind. This usually happens with dozens of iterations of storyboards and wireframes on paper.
The Big Picture is a tool that allows you to visualize the structure of your HTML mock-ups, as you're coding them, in a persistent site-map drag and drop interface.
You can also attach notes to your pages, as a to-do list / comments.
It evolves as you're coding, so you always have an updated version of the big picture.