Ok, many people don’t do that now a days. But, it is probably the best posture for your body. Your body must be used to sitting and sleeping on the hard ground. The reason is that the bones of the body are then used to bending at their most. I know a person who can not sit on the ground and I tried to inculcate that habit. Now, it is easy for the person but a regular practice is necessary otherwise the joints become stiff and body refuses to obey properly and sitting on the ground becomes very difficult.
Move the bones of your body. It is also helpful in arthritis and osteoporosis. There is even a condition called osteo-arthritis in which the lubrication of the joint reduces and joint pains start so avoid that by giving regular exercise to your muscles and joints. That is why it is advised to use the stairs as it increases the bending of the joints in the leg.