The marabou's long throat pouch connects to the left nostril rather than to the gullet, so it fills with air rather than food, and a second inflatable sac sits at the back of the neck. The pouch hangs conspicuously when inflated and shrinks when it is not.
Its documented functions are social and thermal. The pouch is inflated in courtship and dominance displays, and it appears to help shed heat, since the bare skin of the head, neck and pouch offers a large uninsulated surface for a bird standing in African sun. The baldness of the head has a hygienic explanation as well: a feathered head pushed into a carcass fouls, and bare skin cleans more easily, the same solution vultures reached.
The throat pouch is regularly described as a food store, which it is not. Food goes down the oesophagus. What remains genuinely unclear is the relative weight of display and cooling in the pouch's evolution.