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For your added edit question: the publisher and subscriber from the tutorial are in different nodes, thus they each have their own spin call.

Generally speaking, every ROS node has a spin call. ros.spin() is a blocking call that doesn't return until stopped, using that thread of execution to process messages from the callback queue. ros.spinOnce() processes one message in the callback queue then returns; typically, it is placed within a loop that throttles how often it is called.