In a recent exercise with the Open University (where I did my MBA many, many years ago) I was asked the questions “What is a blog?” and “Why do people blog?”. They found it such a useful discussion that they asked me to film the main points and the result is a short video (less than three minutes) which you might find helpful.
http://www8.open.ac.uk/platform/news-and-features/kim-tasso-blogging
They also produced a rather splendid mind map diagram of the reasons:
