Because 360 degrees aren't enough when converting from VR video to normal video and you want a circular fisheye effect.
Bigsh0t is not compatible with parallel processing when exporting video. That option must be turned off, or you will see tearing when using keyframes.
2. Downloads and Source
3. New Features
Added a fisheye parameter to the Equirectangular to Rectilinear plugin. The FOV slider now goes to 720.
Optimized the Equirectangular to Rectilinear plugin to use a cached displacement map.