They're a part of the composite "keys" tied to managing your personal identity. If you use one email for everything, having that email exposed means that a port

They're a part of the composite "keys" tied to managing your personal identity. If you use one email for everything, having that email exposed means that a portion of that key has been compromised, and you may be targeted by attackers that previously did not know of your existence. If you use a unique email for everything, attackers need to determine email+password+whatever, instead of just password+whatever, where "whatever" is security questions or compromising 2FA.