As projects evolve requirements will change. This also hold for editorial content. Structural changes of existing contents will become necessary. Changes in the order of XML elements, removing elements or adding new elements with default values are handled automatically by the "content correction" mechanism of OpenCms. But more complicated adjustments - like renaming elements, changing element types or nesting existing elements into other elements - can't be handled automatically.
Schema versioning allows to adjust existing XML contents to changed schemata via XSL transformations. All adjustments possible via XSLT can be applied and thus various complicated refactorings of schemata become possible, even if productive content already exists.