Given how complex a successful emulator project tends to be, I expect the answer will be highly personal. In my case, it's a combination of nostalgia for the games I grew up playing, curiosity and respect for the developers who were working with such unusual constraints (by today's standards), and a strong desire to help preservationist efforts. I want to help ensure that future curious minds have a wide variety of tools available to play older software once the hardware becomes difficult to acquire. Well, that's the philosophical answer anyway; the more practical answer is that I simply enjoy the challenge, and nostalgia is a great spark to motivate the effort.