We did this pose in one of my classes this week. A student commented how great her back felt while in this position. It's very simple yet can be a difficult pose to maintain. However, the benefits are substantial if you work up to being in the pose for several breaths. It's a great lower back release, creates more mobility in the ankles and stretches the groin. If your low back is currently in pain or you have an active knee injury, try the modified version in the chair for the same benefit.
Always discontinue any pose that causes pain or discomfort in the joints. If your heels can't touch the ground but you are able to squat you'll still get a great stretch balancing on the toes. You can hold on to a wall or chair for extra stability. If having the heels in the air is too intense, roll up a mat or towel under the heels. You may be able to rest the feet on the ground if you take a nice wide stance as shown in the last picture. The closer the feet are together the harder it is to have the heels on the mat.