This is a fabulous stretch to do to help unwind tension in the hip/low back region- especially if you do a lot of sitting. A tight, inflamed piriformis muscle can cause low back problems and even sciatica. Even if you do not have any pain or tension in your low back area you will find this stretch extremely gratifying and beneficial. You should set aside time every day to make this stretch part of your spinal hygiene routine.
https://www.youtube.com/watch?v=QQNsDUtFkzE