Turning a document into a value another document can hold
Stringifying wraps an entire JSON value in quotes and escapes its contents, producing a single JSON string. The result is no longer a structure to a parser - it is one opaque text value that happens to contain JSON. That is exactly what you need when a field in an outer document has to carry a whole inner document.
This is more common than it sounds. Message queues often define an envelope with routing metadata plus a `payload` string, so the broker never has to understand the body. Webhook archives store the original request as text so it survives schema changes. Database `TEXT` columns hold JSON documents in systems without a native JSON type. And configuration formats sometimes need a JSON value where only a string is allowed.