This is a very useful feature for when you have a class that know how to convert its structure to msgpack (specially from compiled modules) and you don't want to have its contents converted to the binary object by returning the converted msgpack from default.

As an example, orjson has a similar feature, which they call Fragment.

Some use cases include, but are not limited to:

• bypassing valid msgpack data from compiled classes (specially deep classes, with many subclasses, dicts, lists)
• caching msgpack output when you need to serialize a class multiple times on the same or different calls
• appending an already serialized payload to a new payload (such as including it on a list or as a value on a dict)

PS: Sorry for closing and reopening it, I had some failed linter tests and wanted to fix them, but ended up pushing a version without any changes, which triggered GitHub's auto close feature.