For the same reason I wrote DSemu and jsGB: an educational experience. I only half-expected to get an emulation to the state where it would emulate commercial games and/or professional demos; the main motivation was to understand how a system works by simulating it, and to gain experience at multiple levels of the architecture at once.

Writing a system emulator in JS/HTML5 is a great way to learn the system being emulated, how to quickly and efficiently read assembly and/or opcodes for the processor under emulation, and acts as an introduction to HTML5 with it.