This is a requirement in some jurisdictions (double opt in) for some cases. Not sure about "User has a confirmed email + account at our service, he wants to sign up for a reminder" use case though.

"Double opt-in"is a spammer term for what normal folk refer to as "confirmed opt-in".

maybe to confirm that the email is valid?

Correct! With schools and students there is always incentive for shenanigans. If I didn't test for emails they could just book-block all teachers.

There would be other ways to clog the system using trashmail providers but thankfully no student cared enough for that yet