I get this question often. I think I am still learning the full answer–and how to explain it.
For now, I will point people to the explanations offered Brad Feld, a venture capitalist and a blogger with several years of experience. In his post “I Blog, I Tweet, But Why (http://www.feld.com/blog/archives/2008/05/i_blog_i_tweet.html)?”, he tounches on many of the points that inspire me.