Actually that is not true:

Concerts could be rescheduled or cancelled, and emails would be the only easy to contact customers for arrangements like refunds.

Similar for online shopping, and in addition, emails could be used for notifying product recalls or class action resolution.

While these cases are relatively rare, it illustrates it is hard to know when an email is "done" -- it could be useful after a purchase itself, or even long after a transaction has finished.