Jeff Atwood proposed this simple rule way back in 2007. Now, as his predictions come true, we may find ourselves asking "why would anyone want to do such a crazy thing?" Well here are three simple reasons:
1: Because we can!
Software development is an art that requires practice. Why do college students write compilers? Why did John Carmack port Rage to iOS? Why did I port Chrono Trigger to a graphing calculator? Software engineers are artisans, these challenges represent our art form at it's best.
3: It shows the power of modern technology.
The fact that we can run a Java Virtual Machine in the browser shows just how far modern browsers and house hold hardware have come. It was only a few years ago that Virtual Machines were a concept reserved for only the most powerful of server farms. The tower of Dubia was not built out of necessity, it instead stands as a monument to human achievement; that is how I view emulator hardware in a browser, a virtual monument of human engineering.
...also, why wouldn't you want to play NES games in your browser?