In Ashtanga second series there are a lot of deep back-bending postures, which can be very pleasant and beneficial for some and maybe too intense for others. Back-bends not only integrate movement in all sections of the spine, but also integrates your limbs while doing the posture. By using the whole body you can avoid over-using your most flexible parts of your body (which could be: shoulders, spine and hips).

Why SHOULD we be practicing backbends

The most obvious reason is that it opens up and stretches out your chest and shoulders. Most people have a tendency to hunch forward as big part of our days we spend by the computer or in our phones. So by doing any variation of a backbend, depending on your level, will greatly improve your posture.
Backbends, especially where you start lying on your stomach, strengthens your back muscles and the spine as you need to use all the back strength to lift the body up from the ground. 
Backbends like Wheel pose (Urdhva Dhanurasana) or Camel pose variations (Ustrasana) are very good for shoulder and hip-flexor stretching. Also most of the time during the day we are bending forward (sitting, picking up something, walking), and by practicing Ustrasana or Urdhva Dhanurasana we open up the whole front of our bodies. 
Also as stated above backbends stretch out the whole front of the body, which improves your lung capacity. Always leaning forward also minimises the lung capacity and the breath becomes short and shallow. By opening up your shoulders and chest you release the tension and give more space for the lungs to expand and your breath becomes deeper and there is a larger capacity of oxygen inflow in your body. 
 

What to think of when practicing any kind of back-bend. 

Some people, when doing back-bends, find it better to squeeze their buttocks and some find it better to keep it relaxed. Some, when flexing, might feel compression in the lower back or at the sacroiliac join and vice versa. It really depends on the personas. We all are so different. So try and see which version is better for you. 
Lower back is the most bendy part in our spines. That keeping in mind, try to intensionally bend more in the upper back. This can take the pressure off of your lower back and will include all parts of spine in action.