Do you suffer from back pain or you have any issues related to your spine?
In yoga the spine is called the Brahma danda, which literally means the walking stick of god.
The spine holds the central vertical nerve centre, the sushumna. The discs between the vertebrae are like fluid filled pillows and are dependent on water and need to be hydrated continuously. They regenerate during the night when we are asleep.
The discs act as shock absorbers for the spine, this is why their presence is extremely important as the spinal column contains and is the conduit of important nerves towards all the organs in the body.
Application of pressure on the discs helps with the rehydration and recovery of them. The proteoglycan structure of the discs works on the basis or osmotic pressure. Do you remember from the physics class? This means that repetitive application of pressure and relief have positive effects on the discs. Through the process of squeezing and soaking, the discs absorb nutrients and fluids as a sponge does when pressed and released. Called imbibition, this process prevents the discs from losing resiliency and becoming narrow. Movement stimulates imbibition by compressing and releasing the discs. Moving the spine in all possible planes – forward, back, sideways and in rotation, is a means to maintain its flexibility and mobility
Yoga asanas with repetitive forward spinal bending and back bending, like e.g. the surya namaskara – sun salutations, therefore are excellent exercise for the health of the spine. The primary aim of any forward folding poses is to stretch and lengthen the lower back. 
Iyengar suggests the following yoga poses for backache: sirsasana/headstand, sarvangasana/shoulderstand, all standing poses, supta padangustasana, janu sirsasana, paschimottanasana/seated forward fold, ardha matsyendrasana, malasana and many more.

Often, strengthening of the lower back goes hand in hand with the stretching. Also keep in mind the engagement of the abdomen in any forward folding pose in order to protect the discs.

References:
1.) Yoga journal
2.) Donna Farhi, Yoga mind body and spirit
3.) BKS Iyengar, Light on yoga
4.) Leslie Kaminoff, Yoga anatomy