There are plenty of ways the innocent could deploy this feature. For example, it’s a great way to turn your headphones into a makeshift baby monitor, or to hear a lecture in a room with less than ideal acoustics.
That said, people are testing the limits of that Bluetooth connection and listening in on rooms for more devious reasons.
Though Apple Support suggests this feature only works with AirPods, any Bluetooth set connected to your phone will work. The feature also coordinates with special hearing aids made for iPhone — aka the devices for which this feature was probably intended originally.
But, like a lot of amazing inventions, it seems plenty of people are finding it useful in discovering cheating significant others, learning what people are saying behind their backs, and more. But as has always been the case for people sticking their noses (and listening devices) where they don’t belong, you reap what you sow.