When you add this post-function to a transition, the add-on simply executes the provided Groovy script against the current issue. This post-function can be used in scenarios where you might want to use and do something (Email issue information, Download attachments of the issue etc.) with the issue information, other than modifying the issue.
You can test your script against any issue using the script tester and debug your script and make changes without having to actually trigger the transition and/or look at the JIRA logs to see the result of the script.