If you would like to see a nice sampling of different ways to use a blog you should check out the blogs hosted by the Z. Smith Reynolds Library at Wake Forest University.  Here's just a couple of examples:

• Spring Break\'08: Arkansas – My first reaction when I saw the title was, "Who goes to Arkansas for spring break?" but once I looked at it I realized that the blog was put together by some students who did a service project in Arkansas over spring break.  Excellent example of a project-based blog that has a defined life span.  I really enjoyed the April 14, 2008 post:
  "I have received great news from Benny that at least 2 of the students
  we worked with at Stephens High School have been admitted to and are
  planning to attend college.  Its so incredible and inspiring to think
  that in a few short days this type of difference could be made. 
  Imagine what would happen if we, as a society, made long-term
  investments in our youth.  It might just change the world!  I challenge
  each of you to think about how you might do that and if we work
  together, maybe we’ll see social justice prevail."
• Research Hacks – A blog that hasn't been updated since December '07 but still has relevance.  Just goes to show that a blog doesn't have to be continually updated to have value.
• Library Suggestion Box – I love this idea: a blog that is clearly labeled as a suggestion box for the library.  Every company should do this, and I think I've found the worlds easiest answer to "Why should my company have a blog?"