When sitting in meditation, you should try to sit so that your thighs are sloped from hips to knees in order to tilt the pelvis forward. This position will support the lower back.
Using a Zafu - whether it is round or crescent-shaped - will cushion your sitting bones and raise the height of your hips to create this desired angle of your thighs.
Placing a Zabuton cushion under the Zafu will cushion your knees, ankles and tops of your feet. This will help release your body of any physical discomforts.
When using a Zafu, a person would sit cross-legged. There are several cross-legged positions to choose from, depending upon your personal flexibility. You could sit in full Lotus (each ankle rests on the opposite thigh), half Lotus (one ankle rests on opposite thigh), or Burmese style (feet are crossed, resting on floor, with one shin in front of the other).
A kneeling position is an alternative to the cross-legged meditation position. This is sometimes referred to as "seiza". For this position, a bench is recommended. Or one could use blankets, cushions, or a yoga bolster between the calves and thighs to gain the proper height and slope of thighs.
You could also sit cross-legged with the meditation bench, if preferred. A zabuton can be used with a meditation bench. Either folded in half (as seen in the picture above), or laid flat with bench on top.