It's a great envelope for structured data, which is likely to be the most common content sent via XML-based messaging schemes. It is simply specified in the spec as an option. If you are referring to encodings in general, there's no way around it.
Different platforms have different binary representations for the same data.
You can't send text or data and expect the receiving end to understand unless both ends agree upon a defined set of supported encodings and both ends can unambiguously identify the specific encoding used for the transmission. It's an unavoidable topic.