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The purpose of a watch stopper is to extend the life of the watch cell in a quartz battery operated watch. When you purchase a new watch, many times the watch will come with this plastic piece on it which must be removed for the watch to be able to start running and keeping time.
If you own several watches you cannot wear them all at the same time. Pulling out the crown will extend the life of the watch cell or battery. Doing this without putting a watch stopper in between the case and the crown will not guarantee the crown will stay out. It can be bumped while putting it in storage and the watch will start running again. Therefore not saving or extending the life of the watch cell or battery. Inserting a watch stopper will prevent this from happening. When you are ready to wear the watch just pull the watch stopper off and set the time.