I love to learn something new, and I'm just as willing to pass on my experience. Writing a tutorial has another advantage for me: to be able to explain even details correctly, I have to go deeper into the subject matter. The finished tutorial can then serve everyone including me as a (hopefully) helpful reference.
The selection of topics depends on its relevance: if several articles already exist on a subject, I don’t have to write another. Common topics are new TYPO3 features, like Routing or the Form Framework. These are supplemented by more general topics of web development.