There is another benefit to tankering not mentioned here.
Airport A raises its fuel prices above the normal.
Airlines flying to airport A start tankering fuel from other airports - Airport A has a glut of fuel and almost no sales
Airport A lowers its fuel prices to slightly lower than normal
Airlines flying to airport A refuel at airport A and tanker to their now more expensive airports.
This is straightforward competition at work to keep aviation fuel costs low. And as stated above there are other advantages for tankering in some situations/operational concepts