Mandating xsi type attribute in a xml schema
The W3C XML Schema Datatype Specification defines numerous datatypes for validating the element content and the attribute value.
These datatypes are intended for use in XML Schema definition and other XML-related documents.
Even in these cases, some description of the web service is needed in order for clients to correctly construct request messages and destructure responses.
Ideally that description would still be WSDL, otherwise clients will have to learn to read and understand multiple description languages.
However, in many cases, the code that is to be exposed as a web service already exists.
In other cases, developers are reluctant to start with WSDL, preferring to start with some programmatic data structure.
Distinguishing between element and attribute names for each namespace is essential when you use schemas from more than one namespace.