Why do we need a disconnect in the grounded conductor?

This same requirement is in Section 230.75. The idea in both sections is to simply have provisions of disconnecting all incoming supply conductors from the premise wiring at the structure be it a service entrance or feeder from a separate structure.

And as the description in the handbook states, this can be done by manually removing the grounded conductor from the bus it is attached to commonly referred to as neutral disconnect link.