DOI's are required (if available) if you use APA style, and are valuable tools for your Zotero library (EndNote, Mendeley, etc.) as you can paste in a DOI and have it automatically create a bibliographic record for the article, dataset, chapter, or other resource type.
DOIs can be an important tool for alternative methods of promoting and determining the spread and impact of scholarly work ("altmetrics") because they provide a common reference link. And you can search by them to get to the article/chapter/etc.
DOI example: 10.1186/s13059-014-0424-0 (they can be longer or shorter, and include letters)