You may run into a scenario, where the value of one schema element is somehow related to the value of another schema element (or even of many other such elements). When content is edited in the content editor and the editor field that displays a schema element changes, it may be valuable to adjust the value displayed in another editor field.
An example is copyright information on images. Your content may allow to insert images and set copyright information on such images. If a picture is exchanged, the copyright information should be adjusted accordingly (possibly read from a property of the picture).