In its basic form, a watch does nothing more than keep the time. However, for those who either demand more from their accessories, or who need one with better functions, a watch can contain extra features, such as a compass, the day of the week, the ability to glow in the dark and the ability to determine the phase of the moon.
A moon phase watch, simply put, is a watch that has a separate display that shows the current phase of the moon. However, there are several different types of moon-phase watch.
Electronic watches may switch the display to show different modes. One of these modes may be the moon phase, which will be displayed and named digitally.
The more common type of moon phase watch is the mechanical variety. There is a separate set of gears in the watch that turns the moon display, covering a certain portion of the moon based on the phase it should be in.
Not many people would find a practical use for a moon-phase watch. Some fisherman who depend on moon phase to understand the tides, and perhaps astronomers would have this need.
The main reason that a person will have a moon phase watch is for show. It makes an interesting feature on a watch, and may be seen as a mark of an expensive or elaborate time piece.