Tell me about Suppression fields, especially Do Not Email

I’m curious about what the Suppression flag for “Do Not Email” on the Contact Record is intended for. Is it to sync requests to not get email from an external DM/TM database, maybe? 

In working for a client investigating unsubscribes, we learned that the Do Not Email field, when checked, triggers a process to change the Email to Unsubscribed. We learned that when working to direct subscribers who wanted to re-subscribe to a Confirmed Opt-In form where they opted back in. Unfortunately, they were again unsubscribed by the automatic process. 

Since the Do Not Email flag seems to override the user choice to re-subscribe, I’m interested in learning about the history of that field, what it’s for, and why a Confirmed Opt-In doesn’t automatically remove the flag. Seems like a user proactively re-subscribing should also automatically remove this flag. 

Give me all of your lore and institutional knowledge!