Each recirc fan recirculate half of the required amount of air in the cabin.
If the recirc fans were off,or non existent,the bleed demand would be higher since all air source to the mix manifold would come from the packs alone.
The right fan recirculates air from the cabin and the Equipment cooling exhaust.
By keeping the fan in auto,you maintain a minimum of air circulation therefore a cooler temperature...i understand it as a need of a light breeze in a hot summer day
The left is selected OFF as recirculating air from a closed pressurized air conditionning bay wouldnt seem to help cooling the cabin.
Kramer,i guess you dont fly the NG.