Useful to verify if a customer purchased with a fake, spam, or disposable email address. Also, in case they purchased via PayPal and the email address on your system is different than in PayPal. Many people have multiple email addresses that are used for certain functions. If the email address they originally used shows risk factors such as being brand new, you can ask them to verify another email address for security purposes. An aged email address with their name in it or work email is a strong trust factor.