It makes citing easier for you, and finding references easier for your reader.
It's used for any digital object (articles, ebooks, standards, datasets, and more)
You can put a DOI into a Google search and find the article citation.
Put this into your browser search bar: doi:10.1080/10242690701505419
If you're referencing something you got from a subscription database, you don't need to worry about including the URL to the journal, just add the DOI.
Sanico, G., & Kakinaka, M. (2008). Terrorism and deterrence policy with transnational support. Defence & Peace Economics, 19, 153 –167. doi:10.1080/10242690701505419