It's a feedback signal showing success at passing automated spam filters.

It confirms that the address is valid, and most likely attended by a real person.

But there are gazillion email leaks. Go scrape them all with hashed passwords. Active emails have no value.

Leaks may have emails that are defunct, belong to dormant accounts nobody reads, throwaway accounts like mailinator.com, honeypots, etc. This confirms the account belongs to a real human actively reading the email.

You're right that they don't have value but there's still plenty of idiots out there buying and selling useless lists and while they're worthless they have a cost to us as recipients since we have to mark them as spam and delete them.

It reveals that their email was detected as spam by an intelligent piece of meat instead of a shard of silicon.

Email lists with verified activity have a higher success rate and sell for more.