An automatic watch has a mechanism that requires regular motion to keep the inner spring constantly charged. This motion is usually provided as it is worn. Once removed from the wrist the stored energy will only run for 24-72 hours, depending upon the model, then the watch will need to be manually rewound and reset.
Many automatic watches have additional features, or 'complications' - date/lunar phase/world time etc. so the task of rewinding and recalibrating a watch if it runs down is best avoided.
Furthermore, watches of such calibre are precision instruments and like all thoroughbreds they actually benefit from exercise - if left unwound for long periods, internal lubrication can be lost from critical areas which will eventually lead to excessive wear on components and subsequent lack of accuracy.