These are scratch-resistant glass filters that are mounted directly to a GoPro HERO3, GoPro HERO3+ or HERO4 and not used with the waterproof housing.
The advantage of these filters is the ability to slow down the shutter speed of the cameras (since it cannot be adjusted manually) and, therefore, reduce the jello effect on the video image that is caused by the combination of a fast shutter speed and the drone’s vibration. Even if the GoPro is mounted on a gimbal, the lower shutter speed will produce smoother motion and a more pleasing video image.[youtube https://www.youtube.com/watch?v=LAVfnKB2ENY&w=700]
The PolarPro FPV Quadcopter Filter 3-pack retails for $49.99. Check it out here at B&H Photo.
There is also a dedicated kit for Sony ActionCam models.