Schemas for Interchange

If XML is so flexible, why all the committees? Application developers aren't normally fond of complexity, at least complexity that they have to deal with. Many folks want to use XML tools in simple predictable ways, using set vocabularies and structures. These folks tend to gather to hammer out standard descriptions of XML documents, using DTDs or Schemas.
Different points of friction In some cases, the prospect of dealing with multiple XML vocabularies terrifies developers and promises dangerous chaos. In other cases, the prospect of committees spending years trying to reach agreement about what order should look like promises dangerous delay, and the prospect of mediocre results.
Understanding the limits of particular documents Even if committees aren't your thing, schemas and DTDs do an excellent job of setting limits on document structures. Working with documents where these limits have been formally established is more predictable than working with documents where no formal limits are provided.