Text formatters in this general style, often known as "runoff programs" after the original one at MIT, are in no way obsolete. Modern computers can run WYSIWYG editors, such as word processors and desktop-publishing systems, and those are used by most people, but runoff programs are very useful for long and complex documents with structured formatting.
For example, consider the documentation for a reasonably complicated programming API. The formatting is standardised for data types, data structures, objects, functions and so on. Using explicit mark-up, usually via macros in a runoff-style language, allows that standardised formatting to be applied far more easily and reliably than setting styles by hand in a WYSIWYG program.
Changing the definitions of those macros allows you to output the same basic text in very different ways. You could generate a printed manual, or a set of HTML files for on-line help, from the same source file. You can do mass edits on the document with text-processing tools like awk, python or perl. Using a runoff system isn't appropriate for all documentation tasks, but it's excellent for the really industrial-scale ones.