In other words, it can tell you whether the document is valid.
From the standpoint of an application that is interested in processing only the XML data, that is a good thing because the application is never bothered with white space that exists purely to make the XML file readable.
uses the non-validating parser by default, but it can also activate validation.
Activating validation allows the application to tell whether the XML document contains the right tags or whether those tags are in the right sequence.
(Even a DTD qualifies as a "schema," although it is the only one that does not use XML syntax to describe schema constraints.) However, "XML Schema" presents us with a terminology challenge.
Although the phrase "XML Schema schema" would be precise, we will use the phrase "XML Schema definition" to avoid the appearance of redundancy.