When peer reviews are assigned manually, reviewers receive a notification and To Do list item to complete the peer review as soon as it's assigned to them. This is regardless of whether there is a submission for them to review or not.
If the student being reviewed has not completed a submission, the reviewer receives a "No Submission" message when they attempt to complete the peer review. This creates confusion for the reviewer, embarrassment for the student being reviewed, and questions for the instructor/Canvas support team.
For instance, an instructor may want to set up peer reviews in a very specific way for a large class before an assignment is due. When she assigns the peer reviews, notifications/To Do list items are immediately deployed. Students then attempt to complete the peer review and become confused as to why there's nothing for them to review.
Instead, notifications and To Do list items should only be activated when the student being reviewed has completed a submission to the assignment.