Comment by layer8

Namespaces are essential whenever you want to insert contents defined by one schema into “payload” or “application-defined” elements of another schema. There are also more complex scenarios where attributes from one schema are used to annotate elements from a different schema.

Well, I guess we could do it like libraries in C-land and have every schema add its own informal identifier prefix to avoid name collisions. But there’s a reason why programming languages moved to namespaces as an explicit notion.