On top, Visual Testing extends functional tests with implicit tests, i.e. test that come for free. TestResults.io calls this feature Human Factor: Imagine a tester who executes the same test case 10x times over the development time of an email application. On the 6th execution the functionality of sending an email still works as expected but your tester notices that there is a new line added to the sent email every time. While this was not part of the test case based on his previous knowledge, he noticed a subtle change. It turns out it found a new bug.
This is exactly what the Human Factor does. It keeps track on how your application looked to the customers and checks if there are unexpected differences. Try this with your current pure functional test automation.