Why has XHTML been deemed as necessary?

XHTML was created to replace the old syntax system of which HTML was based off of; the Standard Generalized Markup Language (SGML). SGML was difficult for computers to read and manipulate automaticly -- therefore, a new HTML system involving XML was proposed. By conforming HTML into XML syntax, computers were able to read and manipulate standard XHTML (the new name for the combined-syntax language) with accuracy.