It is not just for storing variations of a prompt. It separates context to make intentions clear. Here is a XMl prompt serialized from PromptML program:

https://gist.github.com/narenaryan/651d8081eaaffa846e05da7a3...

You can test it with Claude and GPT-3.5 or GPT-4o, and they all will bring strikingly bring similar results with differences in detail.

YAML & XML serializations are coming in v0.6.0 of PromptML.