As for your question: you would have to ask the person who decided to do it that way why they made that decision. It's possible that it was a good decision, or that it was a bad decision, but without any details it's useless to speculate about it.
- easy to create/modify example data with a text editor so you can test data processing independently from data creation
... see for example the use of SOAPui to manipulate SOAP XML messages, especially for creating error conditions
- easy to read actual data transmissions - see SOAPui again
- easy to add new data elements without having to modify existing programming (assuming you chose a good structure in the first place.)