Another reason: If a friend sees me on the street, it's very simple to turn around and go talk, much easier than in a car. This happened to me the other day, a friend yelled out my name while I was riding past, and I made a quick 180 turn and went and said hi. I guess this equates to greater maneuverability, and an easier time connecting with other people who are also traveling by foot or by bike.
And another: Bike ciommuting for the last 3 years, I have gained an much more thorough knowledge of the topography of my town. It's one thing to know that a certain street has a hill on it. I enjoy the bigger picture of riding enough streets that I can view a town-wide topographical map in my mind.