Even if you set practice quizzes, for example, with a due date, that date doesn't appear on the calendar. The only place for students to see that dates is if they go through Modules (our institution doesn't allow Quizzes to be shown in the navigation menu, and Practice Quizzes don't show up in Assignments). 

This would be not ideal, but doable, except there are times when dates only show up in the Calendar (Calendar Events). 

All of our courses have practice quizzes (for a code of conduct quiz), and all of our courses have calendar events (to denote when discussion response posts are due, because discussions only allow for one due date).

So we have a situation where there's no one place for a student or an instructor (or instructional designer) to go to see all the dates in a course. 

Edited to add: it is not obvious that the dates for practice quizzes do not show up on the calendar. When you set these dates, there's no warning that they won't appear on the calendar. So an unwitting instructor could be assigning this quiz and think they're giving students a due date that will appear on the Calendar, but it doesn't. 

Much better would be if everything with a due date appeared on the Calendar. The calendar is no longer a place just for dates for graded assignments, since now there are calendar events and page to-do dates. All due dates should appear on the calendar.