The fans save fuel by recycling cabin air back into the mix manifold. With the fans off the packs conditioned air requirement would be higher which would thus use more bleed air and burn more fuel.
Its my company SOP to have the fans on whenever PAX are on the A/C. Pre conditioned air would still cool the cabin without them but the fans will help increase the circulation of air. Some pilots will switch off one though due to the noise it makes in the flight deck!