DocBook provides for more than just formatting. You can automatically build indexes, tables of contents, and links within the document or to outside. The Jade and OpenJade packages also let you export (I'll call it render from here on) DocBook to LaTeX, info, text, HTML, and RTF. From these basic formats, you can then create other formats such as MS Word, PostScript, PDF and so on. Programs like LyX allow you to write in TeX format, then export it as DocBook SGML and render from SGML to whatever you chose. In the end, DocBook is more concerned about the way elements work instead of the way they look. A big distinction,and one that will let you write faster, since you don't have to worry about placement of paragraphs, font sizes, font types, and so on.
There are other DTDs than DocBook, but there are a few reasons not to use them. First, DocBook is the most popular DTD, being used by more than a dozen major open source projects from GNOME to Python to FreeBSD. Breaking compatability with those groups would only confuse potential authors. Second, the tools for DocBook are far more developed than others. DocBook support is included in most Linux distributions, allowing you to send raw SGML or XML to be processed at the receiver's end. And finally, while DocBook has an extensive set of tags (over 300 in all), a majority of them do not need to be used for simple documentation. Starting with one of the available templates will allow you to quickly start writing DocBook with minimal experience.