Earlier quoted context omitted.
It's a problem when using email to register into a different site. Services without email validation will happily accept those two addresses separately, because they don't adhere to Gmail's internal conventions. If the site then sends mail to its registered accounts, then the Gmail address will receive unwanted mail.
Why would you register with an e-mail address that's not yours in the first place?
(Examples include both Gmail and other systems.)
https://news.ycombinator.com/item?id=12025091>
https://news.ycombinator.com/item?id=12027598>
https://news.ycombinator.com/item?id=17284211>
https://news.ycombinator.com/item?id=36756201>