quota is another reason. if you operate a few flights into/out of a neighbouring country, you may be restricted to a quota as well.
or same country but diff airport, the more pop one will almost certainly have a quota.
another reason, although, rare, is the turn around time. if your handling agent is the bottom of the bid sort, he may be stretched for a refueller, so you may need to BYO.
on the 747-400, for every 1000kg of zfw incr (in this case fuel) we burn about 100 kgs over 7 hrs of flt.