When would you reshape JSON with a Data Mapper Transform rather than doing it in the OmniScript or in Apex?

Reshaping has to happen somewhere in almost every integration, and where you put it decides how many places have to change when the external contract moves.

1. Use a Transform when the mapping is a stable contract between two structures — an external payload and your internal model, or an internal model and a partner's expected format. Defining it once means every consumer gets the same shape and a change to the external contract is a single edit.
2. Use Set Values in the OmniScript only for small, script-specific adjustments that no other consumer needs. Reshaping heavily in the script duplicates the mapping into every OmniScript that touches the same data.
3. Use Apex when the transformation needs genuine algorithmic work — conditional restructuring that depends on the content, recursive traversal, or heavy computation that formula functions express awkwardly.
4. A Transform touches no Salesforce data, so it is cheap and safe to call from inside an Integration Procedure between a retrieval step and a response, which is where most of them belong.
5. The signal you have chosen wrong: if the same field-renaming logic appears in three OmniScripts, it should have been a Transform; if a Transform has grown a tangle of nested conditional formulas nobody can read, it should have been Apex.