Comment by layer8

This is bad in the case of online orders, where the order will usually go through anyway (vendors want to sell even when you don’t confirm your email address, and don’t care a lot about someone else getting the notification emails), because if by mistake you registered your email address as new although you already had an account with that address, the order won’t get associated with your account. Or if it does automatically get associated with the account (or after confirmation), then that’s bad as well if you really entered someone else’s email address, because your order will then get associated with their account.

This is the reason why attempting to sign up for an existing account generally fails right away, at least for online retailers.