Without air in the bag, the wine is all forced (by atmospheric pressure) to the bottom of the bag, which is below the tap. The bag itself is scrunched around the wine at the bottom of the box. That makes it more difficult for the wine to reach the tap, when the level is low, even if you tilt the bag. As long as the level of the wine is above the tap, there is no problem.
If you let air in, then the wine can slosh around in the bag, so that it can reach the tap more easily. This only helps when the wine level is getting very low, when access to the tap is obstructed. As long as most of the bag (and wine) is still above the tap this has no effect.