One common situation when registering a new account with a service (say my.opera.com) is that it requires email confirmation from you to activate that account. This is part of a handshake, where both parties present their credentials and confirm who the other one is. It is also a neat way for the service to make sure that the user has a valid email (to be blocked if a troll or spammer) which is also a universally unique identification. Two independent parties will never have the same email address while they could have the same username on different services (I am "jax" here, but on other sites someone else could have taken that username).
Unfortunately as often as not the confirmation request message to make sure that the user is not a spammer will itself end up in the user's spam folder since the mail program or service can't know that the email isn't from a spammer. …