why to define ros::spin() in another thread?

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Hey all,

There is an example in actionlib tutorial: wiki.ros.org/actionlib_tutorials/Tutorials/SimpleActionClient(Threaded). In this example the ros::spin() command is used in another thread. Why should I create a ros::spin() command in a new thread? Why shouldn't I use this command in the end of the main like I see in the most of examples? What's the difference between these two cases?

Thanks in advance.