The DEA, for instance, could attempt to locate "meth manufacturers" or cannabis growers by singling out those who call (or are called by) pharmacies or garden shops. The IRS or FBI may choose to focus on banks, casinos, or other "financial institutions" (as they loosely define them).
And gun owners? The BATFE could locate potential "gun criminals" (or identify targets for later seizure) simply by matching call records to and from dealers of firearms or firearm accessories...and you'd never have a clue until they broke down your door in a midnight raid.
You know, it's only Wednesday, and this week has been so full of disquieting news that I'm already looking forward to the weekend.