On export <NULL> indicates of the absence of data in the field that was exported. This is useful because you want to know the difference between blank (which is valid data for some fields) and absent items like missing node assignments, missing parameter values, and so on.
On import <NULL> causes Bulk Edit to generally ignore these data fields. For example:
If you are creating modules with an import and the node_name fields are <NULL> the import creates the modules but does not assign them to a node.
If you are modifying existing modules with an import and the node_name fields are <NULL>, the import does not affect the existing node assignments.
So, <NULL> is useful to avoid having to create a unique format file for each variant of a similar type of object you are importing (for example, each instance of a module can be customized).
In addition, if an import excludes fields for which data exists in an object, the existing data is not affected. For example, if you are updating a module's configuration and the module already had a module_type value, importing a data file that excludes the module_type field does not affect the module's existing module type.