DOI's allow identification of a specific document. This is particularly useful in the sciences and by government and non-governmental agencies as there may be evolving variations of an article or document and the DOI allows the user to differentiate between various versions
Not all publications have DOI's. If there is a DOI, it must be included at the end of the citation. The DOI is a relatively recent development and does not necessarily indicate that there is more than one version of a document.