A feature added on Dec, 7, 2016, announced on GitHub blog, introduced the option to add reviewers to a Pull Request
You can now request a review explicitly from collaborators, making it easier to specify who you’d like to review your pull request.
You can also see a list of people who you are awaiting review from in the pull request page sidebar, as well as the status of reviews from those who have already left them.
However, explicit setting a reviewer for a PR was already done by assigning people (assignees option).
With both options now available, what’s the role of each option since they both share the same end goal?