Yes, I know, it sounds silly, but bear with me.
The nature of acceptance tests is not really to tests units of code, but to assure that the behavior of your application meets a certain set of criteria (Behat Scenarios).
When your applications grow over time, code coverage can be a nice tool to help you pinpoint where you need to add more tests. In a perfect world tests are added while implementing new features so that your applications are always fully tested, but that isn’t always as easy as it sounds.
This post is a follow-up to Using PHP’s built-in web server in Behat tests that was published some months back, so if you haven’t read that one yet make sure to do so before continuing. If you are not familiar with Behat I suggest you have a look at the quick intro guide as well.