You geofence a tight radius around an FBI office. You see a device pop up ~200x in a year. You have high confidence anything meeting that criteria is an FBI phone. But maybe it's the guy who owns the Chinese restaurant across the street or something.

So you take those suspects and now you buy their location data. And you see 3/4 of them show up at other FBI offices across the country. Chinese restaurant guy isn't going to fly to DC to cook in another restaurant next to the FBI building. But who would? An FBI agent.