Depending on how a thesis or dissertation is published, the work may have a fairly limited audience, and it's understandable that an author would wish to share his or her research with a broader community. Articles in scholarly journals and books tend to have a greater reach within academic communities and are therefore an excellent way to share research with other scholars. When approaching adapting a thesis or dissertation for another purpose, be transparent with the prospective publisher regarding how the document was originally published and any copyright considerations. Some publishers may not consider student work that had a limited audience (e.g., published only in print) and that didn't undergo a peer review process as a prior publication, and therefore the revised version can be considered a new document. If the thesis or dissertation was made widely available online, a publisher may consider the work to have been formally published, in which case an author will need to determine with the publisher what degree of information from the original work can be included in the new work to still be considered original work and not in any violation of copyright.
If you are considering a thesis by publication, please ensure that your publication agreements with the publishers permit you to use the published articles or chapters for your academic purposes. See "Protect your copyright" for more information regarding negotiating copyright agreements, and "Publishing your dissertation" for information on how to revise a dissertation for other purposes.