It is both. The structure does actually help LLMs. Interestingly, LLMs seem to respond well to the repetition of XML-like structure (that is, the presence of closing tags), even if it's just an ad-hoc constructed language with no real schema.

Not sure if changed with GPT-5, but Claude was explicitly tuned to leverage XML-like tags, while GPT-3.5 and 4 seemed to prefer Markdown-like structure.

https://docs.anthropic.com/en/docs/build-with-claude/prompt-...