Here is a paper
(PDF) by Bonnie Nardi, Diane Schiano,
Michelle Gumbrecht and Luke Swartz about motivations for
weblogging, quite interesting to read:

"We discovered five major motivations for blogging:
documenting the author's life, providing commentary and
opinions, expressing deeply felt emotions, working out ideas
through writing, and forming and maintaining communities or
forums. These motivations are by no means mutually exclusive,
and can come into play simultaneously."

(Post originally written by Ruslan Spivak on the old Nuxeo blogs.)