I love L shaped yoga poses. One of the hardest things to do in yoga and in life is build muscles in the back. Personally I have had a decade long struggle with my latissimus dorsi, a long and broad muscle of the back that connects the pelvis and the spine to the arms. I have been trying to bring tone to the muscle for a long time without much success.
I think everyone leans backwards when standing, shortening and tightening key muscles of the lower back and spine. The L shaped yoga poses, handstand, forearm stand and headstand all work to address these muscles specifically. Most of our postural imbalances are magnified when we go up into the l shapes and the work is an attempt to shorten the front of the body while trying to lengthen the back of the body.
Technically, it is important to establish a straight line from the wrists, or elbows, to the hips. Keeping the shoulders in this line makes the poses a lot harder but by stacking the bones correctly we allow our muscle to do the work instead of sinking into and straining our joints.
These are preparatory poses for the full upside down inversions. Once you are able to do yoga poses such as the L shapes, it shows that you have the stability in the shoulder girdle to support the weight of the body on top of the arms.