These signal captures are paired with millions—or maybe billions—of Wi-Fi “snapshots” taken every day by iPhones, iPads, and Macs that help flesh out the picture. Your devices are constantly scanning for nearby Wi-Fi networks, even when connected to an active Wi-Fi router or hotspot. Those scans contain signal strength information and bolster Apple’s database.
*Interesting read. Data can be collected and put to use in ways you hadn’t thought of.