To reap maximum benefit you must practise asanas in their true spirit. There is a science to each posture. Therefore, when you practise an asana do it properly as per directions. Wrong interpretation and practise has serious side effects.
Among these 84 primary asanas, if you can master the practise of siddhasana and padmasana or either, sound mental and physical health is guaranteed. The key 84 asanas are a culmination of years of research and practise by hatha yogis. Each asana has been devised giving careful consideration to the health of every individual part of the body. When we meditate while sitting in a particular asana in the prescribed way, then prana, the life-force flows in each and every cell of the body keeping it healthy. As long as we sit in the desired asana with awareness and concentration, breathing and relaxing correctly, no disease shall dare approach us! But to stay fit and fine, retaining muscle strength and shape you need to practise other specific asanas as well.
Meditative asanas are known to prolong life span. That is why in ancient times yogis who sat in meditative postures lived for hundreds of years, disease- free. Thus the yogic asanas affect not only the physical body but also the astral body, the energetic body and the mind. The yoga we practice today is a poor shadow of what true yog is. If you introduce yoga in your life in the spirit it was intended, pretty soon hospitals will become obsolete, used only for emergencies!