To answer this from the taijiquan perspective, the stability comes from being properly balanced on one leg. The second leg is a spare for when something happens

To answer this from the taijiquan perspective, the stability comes from being properly balanced on one leg. The second leg is a spare for when something happens unexpectedly like a gust of wind. By forcing people to keep one leg off the ground, you're forcing them to balance on one leg and learn to be more stable like that. If you have no balance, it doesn't work. That's why tai chi is often recommended for older people to improve their balance and reduce the risk of falls.