It protects your lower back from going into extension, which protects your spine and engages your core. Tilting your pelvis will also eliminate your lower back from doing the exercise, which is a very common problem, as it commonly takes over for the glutes. This will lead to back pain and low glute development. Tilting your pelvis prevents your lower back from doing the work, and forces your glutes to do the work instead along with your hamstrings to some degree. Also in contrast to your diagram, the hip thrust puts the body horizontally, having the weight all come down on your back in the vertical plane, and you really do not want the weight to all be focused on your extended spine.
Your lower back will also try to naturally go into extension and anteriorly tilt due to the weight, so forcing your back against these forces protects your spine but also allows the weight to do its job rather than just crush your spine. Anytime you are trying to fight extension, anti-extension exercises or movements such as tilting pelvis and squeezing abs will help. if you were to try to tilt your spine in the diagram you used above however, standing up, the weight comes right onto the tilted spine which doesnt protect it, and will just help the weight give you a herniated disc. In this instance, the weight is pushing down on your entire spine rather than just a part of it, and its much more difficult to protect your spine.