Its actually to increase motion blur.
In the cinematic world, you always want to set your shutter speed to be 1/2xFPS or 1/FPS. So, if we're shooting at 60fps, we want a 1/120 or 1/60 shutter speed. If shooting at 30fps, we want 1/60 or 1/30 shutter speed.
With a fixed aperture camera such as a GoPro or the one in the Inspire / P3, the only way you can cut the light down to get to your desired shutter speed is with neutral density filters.
In the cinematic world, you always want to set your shutter speed to be 1/2xFPS or 1/FPS. So, if we're shooting at 60fps, we want a 1/120 or 1/60 shutter speed. If shooting at 30fps, we want 1/60 or 1/30 shutter speed.
With a fixed aperture camera such as a GoPro or the one in the Inspire / P3, the only way you can cut the light down to get to your desired shutter speed is with neutral density filters.