Some citation styles require or encourage the inclusion of a DOI in your reference entry.
A DOI® (digital object identifier) is a unique number, that is assigned to identify a journal article, book chapter or data set, and provides a persistent link to its location on the Internet. Not all journal articles have DOIs, particularly older journal articles and articles from lesser-known journals. However, more and more articles are now assigned a DOI.
DOIs usually appear on the first page of the article, or they can appear on the page of the database which has the full text article linked.
To find out if an article has a DOI, you can use a Metadata search engine like CrossRef https://search.crossref.org/