Why are the photos and videos distorted on the margins?(YI 4K+ Action Camera)

a. YI Action Camera uses a 155°ultra-wide-angle lens resulting in some distortion.
b. You can go to "Settings" - "System Settings" - "Lens Distortion Correction" and turn on "Lens Distortion Correction."  Doing so will make the photos or videos look normal . When you turn "Lens Distortion Correction" off, the photos or videos will appear distorted again.