Electricity costs less than half of what gas costs. So, the more the vehicle operates in electric mode, the more money you save. Most people will be able to do daily commuting without using any gas.
Oil, airfilters, spark plugs, and brakes will last longer than in a gasoline-only vehicle since the plug-in hybrid will mostly operate in electric-only mode.
Electric motors provide instant torque, which means there is no delay in power from a stop. With no waiting for an engine to rev up, city driving is a breeze.
Make fewer trips to the gas station. Since most people's commute is less than 20 miles, daily commuting can be done without using any gas. No more waiting in line at the gas station multiple time a week
Never worry about how far you can go. Since plug-in hybrids have a gasoline engine, they can be driven just like regular cars. When the battery is drain, just keep driving on gas.
This three minute video by the Department of Energy does a great job of explaining the benefits of a plug-in hybrid vehicle.