The image shows the seat is slightly higher than the back of a persons knees affording legs to slope gently down. This posture enables gravity to assist your circulation by pulling denser blood, lymph and other fluids down towards the ground, which in turn generates a return flow back up the body. A simple flow and return system that assists the heart to drive the blood around our body has given us the ability to be ahad of the animal Kingom. When we stood up we did so because it felt good, it felt right and it changed our circulation completely.
Watch any new born animal in the wild or captivity and you can see it intially struggling to stand up. Yes it needs to do this because of predation but a new born has no knowledge of predaters so it stands up because it feel correct to do so. Initially the animal is wobbling around trying to stand erect and within a few minutes the hydroulics kick in and the animal is walking and in somae cases can run alongside it's mother. One could argue that human babies are more dependent on us because they spend many months being unable to walk, but you might be surprised to learn that babies in Africa have walked at age 5 months and there are cases in the UK such as this one. Yet in the workplace sitting at a desk our posture is not great, sitting in a car for example we are forced into wheelchair posture, which leads to people falling asleep at the wheel while driving.
Get our sitting posture wrong and we place not only our lives and health in danger. I have conducted experiments to confirm that simply raising ones seat in a car so that your sitting posture resembles the first image in this set of three and you can eliminate tiredness, get out of the car after a very long journey without feeling adverse affects including stiff legs and ballance issues. Soi something is not quie right with the car seat designs. However, many commercial vehicles afford the driver to sit fairly close to the image shown and I have driven trucks over long distances and noticed not feeling anywhere near as tired as when driving a car.