Basic Use Cases

Minimal transformations Given this detailed flow of information, it's possible to tell a processor to, for example, change namespace values and prefixes in a document without forcing expansion of entities or loss of CDATA boundaries.
Selective erasures Ripper makes it easy to do things like remove CDATA sections from the content of particularly chosen elements.
Sugar-based processing While XML features like whitespace in tags and quoting style are often derided as "syntax sugar", they also give developers an opportunity to mark content without changing its official structure. With a framework like this one, a developer might well make changes to all values of the href attribute which happen to be in single quotes, ignoring those in double quotes.
Supplemental information Ripper's approach makes it possible to build filters which add namespace or entity processing information to documents when necessary, something especially useful for dealing with external entities in isolation from their containing documents.