what is the use of namespace in `rosparam dump [file_name] [namespace]` command?

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( https://wiki.ros.org/ROS/Tutorials/Un... ) I can understand the namespace use in rosparam load [file_name] [namespace] command in the example given, but what is the use for it in dump file generation, i tried to understand it by using double tab key, and ran this command rosparam dump roslog.yaml /background_b it ran successfully but when i tried to load it by rosparam load roslog.yaml copy it resulted in error :-

Traceback (most recent call last):
  File "/opt/ros/kinetic/bin/rosparam", line 35, in <module>
    rosparam.yamlmain()
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rosparam/__init__.py", line 626, in yamlmain
    _rosparam_cmd_set_load(command, argv)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rosparam/__init__.py", line 545, in _rosparam_cmd_set_load
    paramlist = load_file(arg, default_namespace=script_resolve_name(NAME, arg2), verbose=options.verbose)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rosparam/__init__.py", line 174, in load_file
    return load_str(f.read(), filename, default_namespace=default_namespace, verbose=verbose)
  File "/opt/ros/kinetic/lib/python2.7/dist-packages/rosparam/__init__.py", line 189, in load_str
    if NS in doc:
TypeError: argument of type 'int' is not iterable

Any clue on what happened and what went wrong?