There's a great post on the Digital Body Language blog which compares authentication to immigration control and is a must read for anyone who hasn't as yet got their head around how authentication can help.
Here's the main concept of the post:
In many ways, this parallels the work of the fine folks who patrol our borders. They employ a variety of techniques and technologies to identify individuals who are not welcome to cross the borders, while allowing legitimate visitors into the country with minimal headache.
Let's look through a few of the approaches that are used.
Your Passport: this is key to your identity, and it is one of the main pieces of information used by the border agents to identify you. In the email world, this is best represented by your IP address. Most of today's deliverability systems use your IP address as the basis of your reputation. Don't share it, any more than you would share your Passport.
Retina Scans: There are more advanced ways of identifying who you are, often involving advanced technology, such as retina scans. Adoption varies, and there's usually a bit of setup and registration you would have to do. In the email world, similarly, there are advanced technologies for identifying who you are, such as DKIM and SPF. Certainly worth doing, however not universally adopted yet.
Criminal Record: Having a clean record is definitely a good thing when you interact with the folks at the border. The same holds true in the email world, one of the key things to look at is an email reputation score that provides the closest thing to a "criminal record" of the IP address you are using. If you are using a partner to manage your emails, be sure to have a look at this.
Police Background Checks: the border guards can obtain a lot of information on a potential visitor by doing a police background check. In the email world, you can run scans and tests of your emails in a similar vein to a background check, flagging anything suspicious or problematic. In the real world a criminal record can be tough to erase, but in the email world, if you see something flagged, there's a good chance you can fix it. Use a Return Path or Pivotal Veracity scan to see what gets flagged before a major mailing, and learn from it.
Visa Stamps: These tell the border guards a lot about where you've been and whether you have been accepted or rejected. Similarly, in the email domain, most major ISPs offer Feedback Loops to indicate whether emails have been rejected, and that there was a complaint. You can then better understand the issue and quickly resolve it. If you're not taking advantage of these, you're missing a key source of information.