When do I need to install python-rosinstall?

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I use ROS fuerte before.

Now I want to use ROS groovy on the same laptop.

I found that I can run rosinstall command now, but I also can install python-rosinstall.

What's their difference?

When is the case to install python-rosinstall?

I run rosinstall:

  sam@sam:~$ rosinstall 

  usage: rosinstall [OPTIONS] INSTALL_PATH [ROSINSTALL FILES OR DIRECTORIES]
  see: http://www.ros.org/wiki/rosinstall

  Common Option:
  -n or --nobuild (don't perform a 'make core_cools' on the ros stack)

  Type 'rosinstall --help' for usage.

  Common invocations:

  initial checkout:   rosinstall ~/ros "http://packages.ros.org/cgi-bin/gen_rosinstall.py?rosdistro=diamondback&variant=ros-full&overlay=no"
  subsequent update:  rosinstall ~/ros


  sam@sam:~$

I run sudo apt-get install python-rosinstall

  sam@sam:~$ LANG=C
  sam@sam:~$ sudo apt-get install python-rosinstall
  Reading package lists... Done
  Building dependency tree       
  Reading state information... Done
  The following packages were automatically installed and are no longer required:
    libgsl0-dev omniorb-nameserver libomnithread3-dev omniidl omniorb libomnithread3c2 libavahi-core-dev libomniorb4-1 libgsl0ldbl
    libomniorb4-dev
  Use 'apt-get autoremove' to remove them.
  The following extra packages will be installed:
    bzr python-bzrlib python-vcstools
  Suggested packages:
    bzr-doc bzr-gtk bzr-svn python-bzrlib.tests bzrtools python-kerberos python-gpgme python-bzrlib-dbg
  The following NEW packages will be installed:
    bzr python-bzrlib python-rosinstall python-vcstools
  0 upgraded, 4 newly installed, 0 to remove and 0 not upgraded.
  Need to get 1,877 kB of archives.
  After this operation, 9,119 kB of additional disk space will be used.
  Do you want to continue [Y/n]?