It seems a little counter-intuitive, but the fact is that I ride my bike to work everyday because I am cheap and lazy.
We all need to get some exercise, but I don't want to spend money to go to a gym, and I don't want to spend any extra time either. Since I have to spend time commuting to and from work anyway, I might as well get a workout at the same time. Plus it saves money that would be spent on a gym and on gasoline.
For the 5 mile commute, it takes about 12 minutes to drive, and about 18 minutes to bicycle. So it only costs me 6 minutes each way. Counting both ways, I get about 36 minutes of aerobic exercise for a 12 minute time investment. Is that geeky or what?
Then there's the price of gas. If my truck gets 20 miles/gallon (which it doesn't), and gas costs $3.00 per gallon, then the 10 mile round trip to and from work costs me $1.50 every day. Maybe a buck-fifty isn't that much to you, but that's about $30.00 a month, which makes a difference to me.
So be cheap; be lazy; start commuting on your bicycle.