I would like to understand under which circumstances it would be appropriate to do this. I’ve found that if I don’t do this and if I also don’t select the UE4 Mannequin skeleton (the Epic Skeleton) upon import of my skeletal mesh into UE4, then I can still retarget a free Epic Skeleton-based animation to my (newly generated by UE4 upon import) skeleton just fine. And the resulting retargeted animation looks just fine.