I work for a few reasons:
web design is the forefront of design as it evolves, if you have an interest in design it makes sense to want to be involved at least to know where it is, if not to help shape trendy
web development is the scene of a lot of innovation when it comes to tools for software development (both for web and beyond) so if you have any interest in software development on any platform, learning about web tech right now is pretty hot
I also enjoy helping people. If I can take something I know, or a skill I have and use it to help 1 person communicate to many people, it multiplies the amount of good I can do. Rather than just helping one person at a time, I can be helping dozens, or hundreds, or thousands - what could be more exciting and fulfilling than that?
to push my own skills and personal development along. I come from a design background with training, but nobody ever taught me how to program, or about computer science. By continuing to work and learn and tackle new problems I can expand myself and become more than I was before.