A dissertation is the main element in completion of a Ph.D. The central element of a doctoral dissertation, and the quality that differentiates it from a master's thesis or an undergraduate thesis, is that it must make an original contribution to its field, usually using primary research. The structure and content of a completed doctoral dissertation is often very different from the structure required for articles or books that are based on it.
When a Ph.D. candidate completes her dissertation, this usually results in three or four copies: one each for the candidate, the dissertation supervisor, the university library and sometimes an archive. Unless a dissertation is subsequently published, these are the only copies that are ever created. What this means in practical terms is that unpublished dissertations are almost never widely read. The vast majority of dissertations serve their purpose of gaining a Ph.D. for their author and then fade into obscurity. If you write a dissertation that you want to have an impact, you will need to revise it and publish it in some form.
One of the easiest options for getting your research into published form is to revise a single chapter into an article for a peer-reviewed journal in your field. The difference between this article and an unpublished dissertation is clear: The article is present in a journal that is printed in thousands of copies and distributed to influential academics around the world. In most cases, the editors of the journal will want the form of the dissertation chapter reworked to some extent to make it more accessible to readers who are probably not experts in that particular subject matter.
Motivated dissertation authors often seek to have their dissertations published in book form. As with journal articles, books that are based on dissertations need to be reworked. This usually takes the form of downplaying the methodology and literature-review sections, cutting down on the density of footnotes and references and generally making the text more readable to non-specialists. A published book can get your name out in your academic field and to the world in general. Having a book and some published articles in your field will be helpful to you in advancing your academic career. Within academia, an unpublished dissertation is really nothing more than a prerequisite.
The Internet has opened up tremendous new opportunities for academic publishing. While having your work accessible online doesn't carry the same weight with hiring committees as an article in a peer-reviewed journal, or better yet a book, it is an effective way to make yourself and your work known, as long as you get it published in the right places. Making contacts through online publishing can be an effective stepping stone toward breaking into journals and book publishing. It's also a useful way to get feedback from other academics about your work.