1. I've been reading through Types and Programming Languages in fits and starts over the last year or so. Operational Semantics is one of the topics covered in the book and I wanted to test my knowledge of it. I did the same thing with Category Theory and jQuery
2. I get a kick out of "teaching" (that's an overestimation of what I'm doing) things like Operational Semantics using examples that developers can relate to out of their own experience. This is also part vanity. I like people to read/enjoy/approve of my ideas, but that requires that they be able to understand them.
3. Writing is a difficult but rewarding exercise. It makes you better at thinking through and conveying your ideas in all mediums.
Hopefully that answers your question.