Authentication is the sum of techniques that authorize email servers to send emails with a domain name.
Anyone may pretend the email address they are using is theirs without asking the real owner’s permission.
Authentication makes it possible for messaging services to confirm that the domain used in the email address is the sender’s.
There are various authentication systems: SPF/SENDER ID, DKIM and DMARC. Messaging services including Yahoo!, AOL, Outlook and Gmail, push the senders to authenticate their emails to get a better deliverability. Besides, some messaging services show a small icon in the mailbox to let you know the sender was authenticated. This reassures the recipient and may impact the opening rate.
Authentication does not guarantee your email will be received even after all that. It does improve your email’s deliverability, but does not mean you can neglect other optimizations required such as the content of the message itself.
Authentication prevents your messages from being filtered which means your campaigns are more likely to get to the mailbox.