[xp84]

I think it depends. The popular exposure to this idea, where you can be accused of self-plagiarism for a paper you write for a class, does seem stupid, because obviously your prof hasn't read your paper you wrote in another class and you're not 'wasting' anyone's time.

I can also appreciate that in a "publishing papers as research" context you're completely right.

[MengerSponge]

Course credit is not the same as professional certifications. You can apply past expertise/work to as many different certifications as you can manage.

Submitting previously graded work for a new course wastes your own time. It also wastes the professor's time, because they assigned that writing for you to develop a specific skill, and you're trying to not practice whatever they're trying to teach you.

If you have a perfectly matched essay to the assignment, you should talk to the professor about how the assignment can be adapted.