The examples they give are getting support and sharing things with another person with an account. As a user, both of those things are easier for me if there is an email associated with the account.
Said another way, the Account needs some human friendly global identifier. The email you use to log in is an obvious choice, and anything else would require extra work from the user to set up. You could have usernames, for example, but that complicates the signup process and still makes sharing things hard. I know my friends emails already, but I don't know what username they ended up with on this site.