I wanted a simple tool to let me play around with mortgage numbers with minimal friction. Other online mortgage calculators that I've seen need requests to the server, full page loads, big pie charts, etc. I don't need any of those bells and whistles.
Instead, this calculator is very snappy because it does everything in the browser. Furthermore, you can "commit" a set of mortgage parameters, which adds it to the list visible below. This allows for easy running comparisons as you change the parameters.