It goes full circle: where does the carbon in the biofuel come from? The plant. Where does the carbon in the plant come from? The air. This is why biofuels are carbon neutral in theory at least. There is of course loss in process like in most things.
In terms of a use of money it is a good way to subsidize the american corn farmer. Whether you believe that is worthwhile depends on your views of WWIII.
The devil is in the details. Where did the land used to plant it came from? What was there before? Deforestation emits a lot of CO2. Fertilizer needs fossil fuels to be manufactured, tractors and harvesters burn diesel, et cetera.
In terms of a use of money it is a good way to subsidize the american corn farmer. Whether you believe that is worthwhile depends on your views of WWIII.