Why do people even still bother with stuff like that?
Is it just for the intimidation factor? If you address an email to me, I'm going to read it, and I am probably going to be too lazy to delete it (at best, it gets a gmail "archive"). AFAIK that disclaimer can't possibly convey any sort of legal obligation for me to obey it.
Not for you to obey it, but it's to protect the sender. For example, if I am offering legal advice to my client and the email inadvertently goes to you I can't force you to delete it, but you could be prohibited from testifying about the contents at trial as it's attorney-client privileged info, and even though it was disclosed to you if the disclosure was accidental then the privilege might still apply. The notice isn't determinative here, but just another layer of protection.
Also mine includes a line about asking to notify me if the recipient received the email in error so I do not make the same mistake again. A nice person would email me back "hey jackass, you sent the email to the wrong person."
I've never sent an email to the wrong person so it's a lot of nothing about nothing.