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.
1. Screenshots
2. Downloads and Source
The source is available at Bitbucket[a].
The software is offered under the GPL 2.0 License[b], which, to quote the all-caps section of the license, means that THE ENTIRE RISK AS TO THE QUALITY AND PERFORMANCE OF THE PROGRAM IS WITH YOU.
3. New Features
Go to Bigsh0t at Bitbucket[c] for an explanation of how the plugins work.
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.