Well, the whole story is in the book, but the short answer is that I was the first information architect in an organization that was traditionally design-oriented, and I felt I needed a tool to help me gain the trust and support of my colleagues.
Ajax isn't a technology. It's really several technologies, each flourishing in its own right, coming together in powerful new ways.
Building technical systems involves a lot of hard work and specialized knowledge: languages and protocols, coding and debugging, testing and refactoring.
User-centered design means understanding what your users need, how they think, and how they behave - and incorporating that understanding into every aspect of your process.
But despite the universality of URLs, we often forget that they're not just a handy way to address network resources. They're also valuable communication tools.