Ah, the beneifts of bike commuting, let's list a few (didn't we do this not too long ago?):

Fuel savings
Insurance savings (I cannot drop ins, cause I still keep a car)
never have to find time to exercise
stay in shape
it's fun
Chicks dig it (not where I live, but I'm sure they do somewhere)
Arrive to/from work happy
Impress the coworkers (not where I work, NASCAR is king)
Freak out the coworkers

And to reiterate (again) what Naisem said, FUEL SAVINGS. The cost of fuel is getting scary. Lucky for us we have an option.