Exactly the same as #includes and #defines in C - they let you organize your code in multiple files, be more concise, and shoot yourself in the foot, repeatedly.
They were useful for document editing use cases - remember this was before SOAP and xml serialization, and sgml tooling that already supported this stuff existed. You can see the record of the decision here:
http://www.w3.org/XML/9712-reports.html#ID5
They were useful for document editing use cases - remember this was before SOAP and xml serialization, and sgml tooling that already supported this stuff existed. You can see the record of the decision here: http://www.w3.org/XML/9712-reports.html#ID5