In short, a lot.
If you're a homeowner you can check the calculations against what an agent has quoted you. If you're an agent, this is a lifesaver for you as it cuts out a lot of the legwork.
For developers, your power comes from the new build toggle button and the advanced filters available within the calculator. Within seconds you can find out the potential value of your next development. If you combine this with the radius tool and the drawing tools you will instantly be able to assess the activity of any of your competitor's developments.