The reason for doing that is to get all of the email in a system, as required by whatever rules apply.
Today I learned about "foldering"[1], which uses the drafts folder of email systems as a "dead drop" between multiple participants. It goes back to at least 2005.
Back when I was in IT, circa 2010, I learned that several of the users of our systems routinely relied on the "deleted items" folder of Microsoft Outlook as a filing system, with multiple gigabytes important files stored there. 8(
Our customs are routinely ignored by everyone else.