Anyone who has hung in with asana practice has observed that tight areas open up over time and poses that were once impossible become possible. While it seems obvious, scientific studies have documented the increased flexibility of muscles and the increased range of movement in different joints that come with yoga practice. When your body is tight, certain areas may suffer – for example a lack of flexibility in the hips can put a strain on the knees. Back pain can be caused by tight hamstrings, that can eventually lead to flattening of the lumbar spine. (source: Timothy McCall, M.D.)
Your physical body becomes more flexible with a consistent yoga practice, but you simultaneously develop significant mental and emotional flexibility through yoga asana.