AeA released a report stating "if everone with the potential to telework did so just 1.6 days per week", 1.35 billion gallons of fuel would be saved.
That's a big number. In addition, this "would prevent 26 billion pounds of carbon dioxide from being released into the atmosphere".
Another benefit is employee moral. If your commute was 40 minutes each way, you would be saving almost an hour and a half. Add to that the reduction of stress from driving/commuting. That's a nice benefit.