I'm having a hard time thinking of a developer documentation use case with any substantial structure that Markdown does support well (by itself).

Any form of structured information, for example API functions and their argument types, properties of a class, or tables of historical compatibility, is already stepping outside Markdown.

You're left with either writing everything out in text and having error-prone parsers cross-check pages against each other, or badly cross-referenced information that goes undetected, or using something that is Markdown plus additional markup for semantic data.