GPS gives you a 3D fix (lat/lon/height). A common use case in aviation, particularly drones, is avoiding controlled airspace. The height of controlled airspace varies with location - for example a rural area might be 7500ft while near an airport traffic zone it might be only 1000ft or lower.
GPS isn't too great in skyscrapers. In most current gen smartphones GPS can only really pin you to a 16ft resolution on a current gen smartphone in ideal conditions [0]. This gets worse indoors and if the target is moving.
For airspace it doesn't matter too much. Allowable Mode-C accuracy is something more than 100 feet (I think) to allow for poor sensors, and controllers simply add a buffer.
If I worked in the advertisement space I'd probably install WiFi networks that could broadcast SSIDs at known mac addresses that are coded to a height. I could install apps on user phones that could read that and pull out height & building name data.