To get a good visual structure in the requirements and to increase maintainability it is necessary to be able to nest FR's.
When describing a functional requirement about for example a new attribute in the logical datamodel, it is often the case that there are other requirements that are directly linked.
For example: when creating a relation input in attribute X is mandatory.
By nesting these requirements the maintainability increases, because if the attribute is not needed, by deleting the main functional requirement the nested requirements are deleted also.
The same in copying requirements, nested requirements are copied also.