It's pretty easy to do something with that info. Let's say there's a pokestop on a military installation, then most visitors are likely to be military personnel, and now we have a list of phones of military personnel. What's more, we can learn which troops are deployed to which bases, and patterns about how troops get redeployed to different bases.
How is this useful? Well, it's well known that Ft. Meade in the US is an NSA outpost. If troops from Ft. Meade regularly get transferred to Camp Pendleton (chosen arbitrarily), then we can infer that there's likely to be a lot of SIGINT (electronic spying) activity at Camp Pendelton. For a smaller installation (say a covert safe house abroad), these sorts of clues may be the first signs that foreign intelligence agencies can use to detect that these bases even exist.
How is this useful? Well, it's well known that Ft. Meade in the US is an NSA outpost. If troops from Ft. Meade regularly get transferred to Camp Pendleton (chosen arbitrarily), then we can infer that there's likely to be a lot of SIGINT (electronic spying) activity at Camp Pendelton. For a smaller installation (say a covert safe house abroad), these sorts of clues may be the first signs that foreign intelligence agencies can use to detect that these bases even exist.