Sometimes the content structure stores semantically similar properties at different locations (due to content evolution or different locations genuinely represent different values).
e.g. say a content represent a real world image as ..../file1.jpg/jcr:content/file. Where jcr:content/metadata/exif:title could be title of image from exif data, jcr:content/jcr:title could be title explicitly set by user.
While displaying a title, it probably makes sense to show jcr:title first and fallback to exif:title and then probaly to node name if nothing else works out.
it'd would be useful to have such overlaying be conveyed to query engine too for lookup, ordering (and, of course, indexing). As a method name, sql colesce is exactly what should serve this purpose.