Email authentication is required so that we can recognise and verify the sender of an incoming email. It is used to confirm that the email has been sent from the source that is stated as the sender of the message. This is important and can be used to fight spam, fraud and forgery. If the emails are not authenticated, then it is likely that they could be rejected if they are suspected to contain spoofing or phishing.
If you receive a message from a large sender (like a financial institution, or a major email provider) that hasn't been authenticated, this message is likely to be forged and you should be careful about replying to it or opening any attachments.