curiouspassion

I find #2 to be very interesting, I can also see how the action of writing can help reinforce other skills/knowledge. Therefore blogging could be a form of self education or improvement.

#1/#3 seem quite specific but I can see how some people may write with those in mind.

The answer you gave (a valid answer) is well known and understood (vanity).

I'm curious why developers blog other than vanity.

What if you don't care for employers or what people think of you? (self employed, etc.)