Node DTO is a module that provides proxy classes that wrap a node for easier access to complex or deeply nested properties.
Such uses would be collecting taxonomy terms based on a vocabulary name. Getting CCK fields without having to walk into field properties etc.
The other big advantage Node DTO provides is an easy way to serialize things into several formats, support for SQL statements, JSON and XML.
This module is used heavily in the XSL Engine for rendering data into XML Documents to be processed by the XSLTProcessor.