Without learning at least the basics of the proper breathing techniques, yoga is no more than a series of somewhat acrobatic exercises. In order to explore the full benefits of all yoga practices you have to realize that deep breathing may bring a new sense to the way you perceive your life and actions. The oxygen we inhale helps the body assimilate the nutrients we eat. While exhaling we are eliminating unwanted carbon dioxide that is toxic for our bodies.
Prana is the name yoga gives to a life force which is experienced as a combination of attributes from both spiritual and physiological elements. The air we breathe is the fuel for this life force. Mastering the intake, output, and internal use of air is essential in order to experience the maximum positive effects of yoga practices.
Keep in mind that yoga breathing is also a factor that contributes to our mental status. A fast and irregular breathing may make our mind anxious and stressed while a calm and deep breath can bring stillness and power to focus.
Maximizing the benefits of this life force, i.e. prana, is one of the things yoga stresses.
Pranayama is the art of mastering your breath to help you achieve above average results from your mind and body. Yogis use Pranayama to prepare for the yoga poses (asanas), and, to achieve a superior state of mind that allows the body to go further "this time" than in the last yoga practice.