Whatever service you use to communicate non-ephemerally on the internet, an email address is needed to do it.
Then during the early 90s, a “hack” was found that created the foundation of most user account systems on the web. That “hack” was the idea of logging into a website with your email address, and proving you were the owner of that email address by having the site send you an SMTP message with a hyperlink back to the site which contained a long code. For the few of us who considered ourselves identity geeks at the time, this did not seem like an approach that would last a long time. Even SMTP seemed like a fad when we already had high end systems like Lotus Notes relying on things like PKI, digital certifications, signing, encryption, etc.
However twenty years later, that “hack” is still the most powerful technique that we rely on to build user account systems. From a purist perspective, emails have some downsides as identifiers. Much of this guide will discuss those downsides, and how to handle them. However nothing else, even phone numbers or social network IDs, has come even close to being as powerful an identifier.