While Derived Answers are one of the last features most authors learn, there are three times when a need can only be answered by a Derived Answer.
- To avoid repetitive processing of the same information
- To perform math functions on the contents of SubLists
- To provide data for a Condition when the data isn't already an Answer
A fourth, less tangible reason is to make the User's task simpler and easier by allowing the document to do more of the heavy lifting.