A writer is a person who would run about in public flapping their arms up and down if it were the only way they could attract attention. H.L. MenckenSome blog for attention, others to change the world. To the hypergraphiliac a blog must be the joy of their life.
But whatever the motive, writing disciplines thought. It requires evidence in support of assertions of fact and coherence in logical development. Writing for publication, even under a pseudonym, reinforces the need for both clarity and focus.
Blogging can thus be important, not so much as a means to propagate one's opinion, than as a way of discovering what one's opinion is.