The recirculation fan system reduces the air conditioning system pack load and the engine bleed air demand.
and
The recirculation fan system circulates air from the passenger cabin around the
lining of the forward cargo compartment. When the overboard exhaust valve is
closed, exhaust air from the equipment cooling system is also diffused to the lining
of the forward cargo compartment for additional inflight heating.
what the book doesn't say:
in addition to the already very poor cockpit ergonomics the recirc fan will deafen you and the uncontrollable air streams might give you a stiff neck.
thats why i like to switch it off. and the increased fuel flow? hey, this poor design is not my fault!
by the way: it's allegedly 20kgs/hour. just one of those numbers you're supposed to comply with - but it's not written anywhere and you can't look it up......