One of the many types of campaigns you can create using Marketing Automation are A/B Testing campaigns. A/B tests allow you to take different versions of the same message (for example, sample A and sample B – hence the name.) Marketing Automation then sends each version to a small sample of recipients to see which one is more successful in getting recipients to open the message and/or click a link inside.
The information provided by A/B testing can be important to fine-tune your emails. In may turn out that one subject line is more effective than another, or that sending at a particular time of day leads to better success. It could also turn out that a variable you thought was important did not actually matter at all.
What Marketing Automation will do is walk you through the campaign setup process and choose how you want to setup your tests. Then, it will collect the results and provide them to you to help guide your future marketing.