Balance exercises help to rehabilitate, restore, and prevent ankle and knee injuries. They work to increase the stability of the joints in the lower extremity. Balance training can also help to prevent falls in the elderly population. Balance training improves proprioception, which is the ability to know where your joints are in relation to one another.
An example of a basic balance exercise is standing on one leg and trying to maintain the position for a certain period of time. To progress this exercise, it can be performed on an unsteady surface to increase the amount of strength/stability required for the exercise. If you have a chronic ankle or knee injury, you would benefit from physical therapy using balance training to improve the strength/stability of your joints.