My experience is people add these signatures because they see other people doing it and they assume it's a good practice or seemingly professional. It may be some inexperienced lawyers recommend it so they have something to say about email policy. It's not hurting anyone beyond eating up mail quotas, right?
“You have no obligation to obey the disclaimer if you decide to read a misdirected email, send it to your friends, or send it to a Wired reporter,” says Susan Lyon, a privacy and data security attorney for Perkins Coie. “You don’t have to worry about that.”
http://www.wired.com/magazine/2010/12/pr_burningquestion_leg...
"This is not an offer to buy or sell securities."
"This is not legal advice."
These are legally useful because rather than enforcing a contract, they explicitly disclaim one.
So everyone puts these signatures/disclaimers on their e-mail now which say (paraphrasing):
> This message is confidential. It may also be privileged or otherwise protected by work product immunity or other legal rules. If you have received it by mistake, please let us know by e-mail reply and delete it from your system; you may not copy this message or disclose its contents to anyone. Please send us by fax any message containing deadlines as incoming e-mails are not screened for response deadlines. The integrity and security of this message cannot be guaranteed on the Internet.
Or similar. Do these things actually have a legal purpose/meaning? I mean can you really enforce a contract the other person hasn't agreed to? Can you really demand what THEY do with an e-mail YOU sent them?
A few years ago I thought this stuff was silly but now a lot of big companies are doing it and I can only assume these companies have a legal department...
PS - If you were to reply I wouldn't assume it was legal advice, I am asking you as a person who just happens to be a lawyer, not as a lawyer. :)