Covert communications could also be an application. Just need line of sight and a signal could be sent undetected, and if anyone or anything above a certain size gets on position to detect it, their detection would be observed by both sender and receiver. The attempt would likely be noticed even before the detection, so the signal could be stopped until whatever it is that’s looking moves on.

Wouldn’t it be far more effective just to put Wi-Fi routers in said street lamps though? You’d almost certainly need far fewer of them for the same coverage.