My theory: people (well, blokes mainly) like to collect things. This is a fact.
A blog is like collecting your own experiences and thoughts, with a mild dose of egoism thrown in for good measure. That’s why they’re so carefully filed using dates, tags, categories, etc, etc. It’s just the male collecting and filing instinct all over again.
If it ain’t alphabeticised CD collections it’s bloody blogs!
I guess this is why the more niche the blog is, the more successful it is: it’s just a collection really.