Obviously, planners themselves have a lot to say as to why. As for me, when I am not a planner (and sometimes even when I am), I am an ardent devotee of evolution. Which usually leads me to ask questions in the passive form. What change is happening to planners when they blog? Does that confer a 'survival value' in any way?
The nature of the question also ensures that I look for broad patterns rather than individual exceptions. And some of the broadest commonalities and patterns we share are rooted in our biology. And possibly because of my viewpoint, the answers I found were steeped in bio-logic.
One final caveat before I begin. In my opinion, what I write here applies to all bloggers, not just planners. I have stuck to planners because my train of thought started with them, and I believe that planners are more keen on both the 'listening' as well as 'talking' aspects of blogging. Which, in my opinion, puts them at the vanguard of blogging.
Enhanced Cognitive Capacity
No, I'm not promising blogging will improve your IQ scores. At least not yet.
One common reason why planners blog is to take notes for self and others on interesting things they have noticed - a scrapbook of sorts. But blogs aren't the first invention to help us store information outside of our limited craniums. So why should they be any different?
I think the difference blogs bring is to the very process of storing and retrieving information. By incorporating reverse chronology, links, tagging and preserving the conversation around each entry (through comments and trackbacks), blogs mimic the very process of memory formation.
Every time you make an entry, it's almost as if you are deliberately firing a synapse. Revisits to the entry and the conversation extending from it fire the synapse again and again - strengthening it in the process. This external synapse is linked to and follows from the internal synapse - the very thought in one's mind - that created the entry.
I believe, over time, as the blog grows it becomes an extended (and integrated) part of our brains - almost like some kind of secondary memory. Of course, the access times for this 'secondary memory' vary wildly (from when we are online to when we are offline, due to bandwidth, etc.) but the accuracy, the vividness and the relative ease of 'recall' make up for it.
(Just in case you are wondering if anything outside of us can be a part of us, Richard Dawkins argues in 'The Extended Phenotype' that there's no reason why an organism should be given an arbitrary boundary of its physical form.)
Of course it follows that the longer (and more) one blogs, the larger this 'secondary memory' grows - and so does a blogger's cognitive capacity. (Theoritically there should be limits where the advantages of this increased secondary memory taper off.)
Biologically, there's an undeniable correlation between brain size and intelligence. This correlation is more precisely measured as brain-to-body mass ratio. And what blogging is doing is adding to our brain size without affecting our body mass - an evolutionary change that bodes very well if past records are anything to go by.
Of course, the results of this enhanced 'cognitive capacity' might not necessarily lead to increased IQ scores. I do think, however, that they will lead to increased storage and processing abilities.
Grow big and remain small
In evolutionary terms, growing big has it's advantages and disadvantages - one can capture prey quite easily but one also needs more food to maintain a bigger body. Staying small means less food needed but a relative difficulty in capturing it.
The spider had found an elegant solution to this conundrum. It stays small but builds a bigger low-cost 'body' - its web - to capture food. Being non-living, the web requires a fraction of the energy a bigger body mass would have needed.
What the spider's web is doing is increasing its 'contact patch' with the environment around it. I believe a similar mechanism is at work in a planner's blog.
A blog and its extended web of comments, trackbacks and links in combination with RSS readers and feeds, maximise a blogger's ability to continually capture the information he needs to feed his 'thought metabolism.'
In my belief, the better bloggers use their blogs as a cachment area for information from the outside world - as comments, links or as a continuation of conversation on other blogs. Which ensures that the energy needed to gather these 'inputs' is just a fraction of what it takes to do it the traditional way.
In fact, I think blogs are the 'long tail' of conversations (and probably even our senses) - carrying on forth both our 'speaking' and 'listening' to places and moments long after we have left the party.
The Dunbar Number and other dominoes
It follows that if our cognitive capacities and brain-body-mass ratio aren't as constant as we thought they were, indexes that depend on these constants will probably be evolving too.
The Dunbar number for eg. I am not quite sure it is a constant or that it is immune to the changes the web (and technology) is wreaking on us.
I have also left unexplored (for the moment) several other reasons why planners blog: the 'Sandbox Universe' where they can hone their thinking and presentation skills, the 'Mesh Memory' system that overlays our collective memories onto one interconnected network and the 'Free Lunch Cafe' where everyone gains by giving away things.