As for the disclaimer in the legal spehere, the primary reason is to try to protect against inadvertent waiver of the attorney-client privilege. To be privileged, a communication can't include anyone but the attorney and the client. Waiver of the privilege is an event with potentially extraordinary damaging consequences - possibly opening up to disclosure in litigation all communications that otherwise would have been privileged on that subject ("subject matter waiver").

But here's the key: there's a difference between intentional and unintentional waiver. On the surface, every email is an intentional act: I affirmatively chose You to send the email to, even if you're the wrong address. That can be interpreted as intentional waiver. But the disclaimer language permits the sender to say, "whoops" - I mistyped/misclicked or whatever - it was unintentional, and the attorney can argue that the privilege should remain intact.