The rationale is that someone (wisely) decided to ask users to confirm passwords because they cannot be seen as they are typed. Then, some eejit decided that if passwords should be entered twice, then so should email addresses, because they are important. Then some nincompoop decided that they would copy these eejits because there must be a good reason for them to do that, so let's do the same.
All these people are completely missing the point that passwords are hidden and can't be checked visually, where email addresses are visible, so then can be, and should be checked by the user.
Asking users to enter email addresses twice doesn't prevent some users entering the wrong address twice, and annoys those users with at least a few brain cells - it is essentially pointless.
If entering an email address twice was of any use to confirm it, why do sites which let you create an account, not ask for your username twice (after all, I need that to log in, just as much as the password), or physical delivery e-commerce sites not ask for your postal address twice, (so my expensive goods don't get delivered to the wrong person), etc.
I've seen people say that entering the email address twice is an 'anti-bot' technique, but that's total nonsense. If a bot can enter an email address once, it can just as easily enter it twice.
If the email address is that vital, then use email address confirmation, otherwise trust the user has entered their email address, checked it visually, and corrected it themselves if necessary. If they're not clever enough to do that, they're probably going to enter the wrong email address twice anyway.