It is important to easily see the tasks that have been completed in the planner in order to keep track what has been done or not. Like if it disappears I would need to find the completed task in the task list, and maybe I do not remember what the task name was. Or lets say I set a task with a time frame, I execute this task but in the end I needed 30min more but I had already clicked that I completed it, one would need to find the task in the specific list and then subtask for example which takes time.

Of course you have the report feature but sometimes ones just needs to visually see what tasks got done by the team quickly without having to read a full report.

Everyone is used for tasks to not disappear from a general calendars, so it is counter intuitive how it is currently set up.

I hope this helps.