Even without a DTD, an XML parser should parse a well-formed XML document. If it is not well-formed it can't be called an XML document. This aspect is good for web applications because the applications need not know the DTD structure used to create the XML document.
Valid XML files are those which have a DTD reference and conforms to the DTD. A valid XML file must also be well-formed. The availability of DTD along with the document facilitates the XML processing programs and rendering of the document by XML-enabled browsers.