What Is the Most Important Technical Reason for Using SGML?
“Abstraction support” — that trait of an information representation (data format, notation, et. al.) that allows its users to distinguish what they consider to be the “abstract” information content of a document from the “style information” that is used to render it, and that allows tools to enforce and preserve that distinction. Abstraction support is what allows SGML documents to be reusable and portable.