The air barrier is intended to stop air from moving through the roofing system. It is generally to stop air pressure from inside the building from pushing up on the roofing material. This is particularly important where there are alot of openings in the walls. On single-ply roofs that have the membrane mechanically fastened an air barrier can also help stop air from being pulled from one area and pushed into another. If you are re-roofing a building there should be some experience as to the air flow and what roofing systems are/were on the building. If the building is strictly climate controlled or could result in negative pressure on the roof then an air barrier should be installed. The air barrier should be sealed around all penetrations.