So what is the use case?
Replacing the PythonScript plugin with a real programming language. ;-)
Notepad++ has some excellent features -- it's got a Boost regex engine, and a simple macro language. However, there are cases when those tools aren't quite sufficient for the task at hand: often, those include things when you need loops (perform the same set of actions many times), or when the search term and/or replacement sequence should be algorithmically generated: in the Notepad++ Community forum, we often see requests for "how to I replace all of these matching numbers with 1 plus the number" or "how do I replace these with a random number or random string" or similar tasks. You cannot do those in the standard Boost-based search-and-replace dialog, but could easily be done in Perl.