Helps to elevate your upper body slightly. It allows you to effortlessly straighten your spine and pull your shoulders back into a healthy posture. It helps to facilitate a proper posture for a prolonged period when you are in a seating position where your back isn’t supported.
The most traditional seating posture on a zafu is the cross-legged lotus position. Here you’d generally sit low and forward on the zafu whereas, in the half-lotus position, you’d tend to sit a little bit higher on it. You also tilt your pelvis slightly forward which helps to keep your spine in alignment.