First, what is an automatic campaign? An automatic campaign looks at your listing content and matches it to what Amazon consumers are searching for. These campaigns require much less care, you can’t add keywords and the bid is set at the ad group level. Very few optimizations can be made, but they can be very effective for two main reasons:
Your ads will only show up on competitor’s listing through automatic campaigns (manual campaigns will not do this). This is important because if your competitors are doing this, you should too. It's another way for you to get additional marketplace real estate.
This is the best way to find new keywords to add to your manual campaigns. Automatic campaigns are the research phase of keyword harvesting. They let you know what search terms customers are converting on, and better yet what search terms they aren’t converting on after clicking.