This allow to create and configure the standard instance of the
external PDF libary, before returning it to the standard writer.
Or, more powerful, this allow to provide a custom implementation
of the external PDF library, allowing for custom behaviors. An
example of that would something like: https://tcpdf.org/examples/example_003/Closes#266
We used to have some kind of wrapper that didn't do much except
forward methods to the real instance. That unnecessary complexity
made it harder to work with the real writer instance.
Paul Klimov