XML is a standard document
Where as Schema is the base document (contains simple and complex type definitions namespaces etc…included) for that XML document you are trying to reference (it’s of a dictionary/DTD for XML document data and validates against) and of course using both you can create an IS document type in the Designer.
I would say Schema is always useful artifact for the XML document for maintaining the document structure wise and validation perspectives.
Also you can google up more for wiki info to get basic precise understanding:)-
If you have both xsd and xml available, you can create a DocumentType in Designer by specifying the xsd as a source.
This guarantees that constraints and type definitions are handled correctly.
These informations cannot be generated when converting xml document to xsd definition.
For further informations check what rmg has suggested.