Why DOI important?
A DOI is an abbreviation for "Digital Object Identifier". If a journal assigns DOI to it's published articles it gives authenticity to a journal. DOI ensure the international standardization of scholarly article/research paper. It helps scholars to easily locate the research paper from your citation, as it always refer to an article/published paper, and only that one ,which in turn increase the acceptance of scholarly data as authorized, lawful and citable material.
DOIs identify content permanently. And as they are coupled with metadata, they can be modified over time to keep track of the locations and characteristics of the objects they identify, both for publisher and authors.
Citations count plays important role in evaluating the importance of published research, so citations to published work matters to all the authors. DOI make sure that each published article get it's unique identify and can be cited easily, which increase the visibility of published work.
So as an author you need to make sure that you publish your research in journal, which provides DOI.