Massage after birth is not only relaxing and needed but also essentially important to get the body back to pre-baby. Our bodies go through a massive change making the connective tissue stretch and is put under strain for over 6 months internally and externally. Your ribs would have been pushed out, your back would have been strained and your pelvic would have moved to make room for the precious soul you carried for 9 months.
If you suffered SPD or Sacral Dysfunction this would have improved post baby but it does not necessarily go completely. Massage can aid in the full recovery of these conditions.
If you suffer from Diastasis Recti again massage can help tremendously in the recovery of this condition. You should obviously do physical rehabilitation for this to improve but if you imagine that the muscles were stretched like an elastic band for a very long period of time. Massage can aid in getting that 'elastic band' back to were it needs to be so that your physical therapy has more of a chance to work.
If you were one of the lucky ones that didn't suffer from any condition or have any issues during labour a massage postnatal is always a good idea just to relax your sympathetic nervous system so that you can sleep better and help your hormone levels rebalance.