Short answer: It's a law called the CAN-SPAM Act which requires email list senders to include an unsubscribe method.

As with all laws though, it just gives the scammers a framework that allows them to dodge the law. The most common dodge I've heard is that they use the fact that you clicked on unsubscribe to confirm that you're a real person so that makes your contact information "verified" and they can sell it to another "business" to still make a profit off you. If they're a more reputable company, the link should actually work and help you remove yourself from the list.

It is a legal requirement.

If you are a scammer there is another reason, If you us it they do know what mail address it it is from so they knew it is an account that is used and someone did read the mail and it did not just end up in a spam filter.

So they can validate that it is an account that is in use and it the address could be used for som phishing attack or just spam.

In addition to some other points. Spam detection systems that email providers use employ a scoring system. For each element of an email that is associated with spam the score goes up. For each element that is associated with legitimate email the score goes down. Things like including a real physical address of the sender’s company, phone numbers, etc are associated with legitimate email. Having a legitimate unsubscribe link helps convenience the spam filter software that the email is legitimate and increases the likelihood that it gets delivered. The vast, vast majority of all email is spam and is blocked outright. Only the “well crafted” spam gets through to your inbox.

Side question, why does every unsubscribe form have the exact same survey? Moat of the times they have identical choices down to the word, like "I no longer want to receive these messages", "I never signed up for this list", etc. No matter the type of content, the country of origin, or whatever, they are all the same. At one point it feels like they're all made by the same software company.

Somehow my email account got hacked in such a way that the hacker subscribed me to literally hundreds of subscriptions. Most are legitimate companies trying so hard to interest me in their products that I have absolutely no interest in. Even with a good third party anti-spam filter on my Outlook, I had to read through each day's long list of spam or risk losing an important legitimate email.

My choices seemed to be either to change my email address (would create all sorts of issues) or just unsubscribe unsubscribe unsubscribe. For the last couple of weeks, every day I've unsubscribed to dozens. The rate of these annoying spams seems to be diminishing but it's by no means over.

What I've learned is that each time I unsubscribe, I get one of about a dozen by now familiar responses: either a web page message that I've been unsubscribed, a request to verify my un-subscription request, or one of several cookie cutter pages asking "why are you unsubscribing?" with the same 5 choices. Most senders stop after I unsubscribe, but a few assholes seem to ignore multiple requests.

I've learned to recognize the unsubscribe link in multiple languages in multiple alphabets. (Google translate is most helpful in this.)

If anyone knows more about this type of hack, I'd be interested in knowing.

Also: some mail clients (gmail, for example) will add an unsubscribe option when they detect something that appears to be from a mailing list. That might be what you're noticing.

They probably use some widely used mailing software to manage sending e-mails to their mailing list/subscribers. Most of these software packages have a setting where you can give the recipient an option to unsubscribe.

Lots of companies that offer e-mail to the standard person (gmail, yahoo) might be more likely to put incoming e-mails without an unsubscribe option on their spam list. If Google puts your company e-mails in the spam box by default, it's much worse (for the advertiser) than giving people the option to unsubscribe.