Basically some software (like 3ds Max or Substance Painter) discard other color attributes besides the first one - so the option to prioritize active_color to be exportred first will be helpful in these cases. It is especially important since currently there is no option to move color attributes around in Blender.
Explained it in details here - blender/blender#104827
To solve that I've added that option in FBX exporter.
Andrej