In today's world we have the capabilities of using great tools and instruments to be able to shape students (and, in some cases parents) in a positive way. The first thing I think an administrator could use a blog for is to allow the community, whether it's the in-house school community or the actual community, a chance to see what is going on inside of their particular school. Some places are notorious for negativity when it comes to the perception of a faculty or the scope in which a school is looked through. Through a blog a principal can put something out to the community that has a more positive spin on said campus.
A second use that administrators can get out of blogging is the quick use of critical information. If an ice storm is coming in or if an extracurricular activity has been postponed they can get the information out quickly and effectively.