Because character definition and solver transferring everything under it, your hierarchy of retargeted skeleton starts under the root, it is good to have the root bone, which serves to position the character in the scene independent of the animation. If you don't want the joint as root or as part of your skeleton, place it under the null. For example in game engines you controlling movement of character by this root bone and animation runs below it. look to project.
happy to help 🙂
MIXAMOvsMOCAP03.c4d