It is a traditional formal way of sitting in Japan, which is also called ‘kiza’ or ‘tanza.’ Originally, it was the formal posture for samurai retainers to prostrate themselves before the lord.
The long hours of seiza involves pain in the foot. For this reason, a zabuton is said to have become popular.
The reason why the depth direction of the zabuton is longer than the width is because the length of the leg below the knee is longer than the width of the thighs when sitting in the seiza style.
It might be good to place the tips of your toes on the outer edge of the zabuton to prevent possible numbness in your feet when sitting in the seiza style.