It enables you to quickly check that input is roughly in the form you expect and quickly reject any document that is too far away from what your process can handle.
If there's a problem with the data, it's better to find out earlier than later.
The You can reuse the same validator and the same schema multiple times in series. Usually the document consumer should choose the schema, not the document producer. All other schema languages require an explicitly specified schema location. The abstract factory design pattern enables this one API to support many different schema languages and object models.
A single implementation usually supports only a subset of the numerous languages and models.
When you pass a URI identifying a particular schema language to that know how to process your schema language.