Linked from
The 13 pages that link to Air density, each with the reason it gives.
Wing loadingRelated: Air density changes the lift available at a given speed and wing area.
Atmospheric dragRelated: Density falls with altitude and directly influences atmospheric drag.
Air massRelated: Temperature and moisture affect density, influencing whether air rises or sinks.
Vertical draftRelated: Density contrasts help determine whether an air parcel rises or sinks.