That is their mission, according to the official website.
"Riding to work on this day is fun and shows the positive value of motorcycling. For many people, riding is a socially responsible form of mobility that saves energy, helps the environment and provides a broad range of other public benefits."
That is their reason, according to Andy Goldfine, this year's event organizer.
In an effort to help make this happen, letters (.pdf link) were sent to the mayors of the 260 largest cities in the United States, which urged them to allow free parking at all meters and municipal ramps on July 18th. In the past, the event has proven quite successful, as the 200,000 or so regular riders swells to about 600,000 on Ride To Work Day. There are quite a few benefits to riding your motorcycle or scooter to work, some of which are lower cost of operation, less congestion and less travel time. According to the Department of Transportation, the average motorcycle gets twice the gas mileage (50.1 avg) of the average automobile (22.3 avg). This means that on Ride To Work Day, 60,000 or so gallons of gasoline will be saved. According to the official site, "If every work day were Ride to Work Day, 15,000,000 less [gallons of gasoline would be used] per year."
[Source: Ride To Work]