When I first started blogging I said that the only possible reason was to say, "Look at me!" My thinking hasn't changed much. I sit at the keyboard typing away at something I hope will strike a chord or give someone that, "a-ha," moment of which I'm so fond.
Over at Firedoglake there's a piece that describes the 5 stages of blogging for those who have never gone through them. Please, take a moment and look to see yourself in that piece.