Not sure if it's a thing of the past, but not too long ago you could use monitor mode to pickup iphones within range and see their AP connection history including, iirc, mac addresses. You could then use wigle to map out paths. Creepy shit.

It's also used for indoor positioning software and pretty sure it's enabled (or at least possible according to vendor patents) on MTA's subway routers/aps. Which again is very creepy if they're using it to track devices with wifi on but not connected.