Mike Ormond's example is a Mandelbrot set which is pretty cool. Wikipedia has an amazing write-up about what exactly a Mandelbrot set is, but it is a perfect example for showing off DeepZoom and the MultiScaleTileSource.
So why would someone need a custom source to do DeepZoom? Not every solution is able to use the DeepZoom composer. I could have parsed images already such as map imaging or a data source that is calculated on the fly.
Both Mike's, awesome job!