We all have different motives for blogging. I would guess that the motive that perhaps unites all bloggers is the desire to reach out to others with similar interests. Dave Jung was the only person at his job who cared about the type of sales and marketing work he does.
"So he did what thousands, perhaps millions of people hoping to reach others with a common interest have done: He became a blogger. A year and a half ago, Jung created his own blog -- short for Weblog-- an online journal, his personal worldwide forum to air his thoughts about marketing and technology and solicit input from others."
"The purpose is to let me express myself, I feel comfortable expressing myself in writing. I have this thought, and the only way I can make it concrete is by putting it into writing, having that invisible person to talk to. It becomes your soap box. At the very least, it allows me to express my thoughts and observations, whether anybody's listening or not."
Dave's blog is done with Blogger. He has several other blogs he runs, most are more personal in nature.