ADE Bloggers is a collaborative blog that has a nice summary of why a teacher might want to use blogging tools. Here's an extract:
"You might like to create a reflective, journal type blog to...
reflect on your teaching experiences.
keep a log of teacher-training experiences.
write a description of a specific teaching unit.
describe what worked for you in the classroom or what didn't work.
provide some teaching tips for other teachers.
write about something you learned from another teacher.
explain teaching insights you gain from what happens in your classes.
share ideas for teaching activities or language games to use in the classroom.
provide some how-to's on using specific technology in the class, describing how you used this technology in your own class.
explore important teaching and learning issues."
There are other answers relating to class weblogs, learner weblogs, shared weblogs, etc...