Umberto Eco says he writes articles simply to get his thoughts in order; in effect, to work out what he actually thinks about a given topic.
Blogger Rohan Maitzen of Novel Readings also noted last week that the value of blogging for him was "sorting out my thoughts more carefully than I sometimes do in a notebook...because of the chance that someone else will read them."
Interestingly though, he goes on to question the value of the blogosphere as a medium for dialogue and exchange. I've always thought that it has been a bit oversold in this respect. Comments are very much a secondary element of the experience, especially for those that get their blog fix through RSS readers. (Strangely, some basic changes to the standard blog interface would I think improve our ability to browse and collate comments and of course print out those posts which do feature a long trail of reader addenda.)
Maitzen thinks that the tensions between specialist and generalist content within the same blog may work to reduce the volume of discussion there, and adds that where there is a level of chatter, it is often"dominated by a fairly small number of contributors, most of whom seem to know each other well and thus to be engaged in their own special game of point-counterpoint."