Testing by "simulating" button presses and other actions like that, including inspecting pixels, is part of so-called "black-box" testing, and offers merit(s) of its own. At least because software is used by people who click buttons which may modify pixels, and these people are not concerned what your model is, they don't even know anything about the way you may have implemented the latter. In the end everything is run on a fairly RISC-y CPU, it either works or it doesn't (from user's perspective) -- replicating the user's workflow is useful in that it it uncovers issues that matter to users and thus normally affect your bottomline.