You'd have to ask Apple, but I'd imagine they would say they use it to improve location services. Access point / MAC address data is really useful in aggregate

You'd have to ask Apple, but I'd imagine they would say they use it to improve location services. Access point / MAC address data is really useful in aggregate as a way of augmenting GPS, especially indoors. And of course it's also a great way of seeing who associates with each other, but that's at least somewhat mitigated by most devices broadcasting random MAC addresses until they actually join a network.