Physical and Therapeutic Benefits

Physical Benefits
This posture is really good for our bodies because it opens up our hips. When in the full posture, it also is helpful in opening up your shoulders. It is good for stretching the groin area of the bent leg and the hamstring area of the straightened leg. It is also very good at building up your core because it takes a lot of strength and energy to descend down, hold it, and come up. Overall, this pose stretches out your hamstrings, opens your hip joints, and strengthens the hips, hamstrings, shoulders, and knees. It can also cleanse the liver and spleen, and improve and regulate the digestive system.

Therapeutic/Spiritual Benefits
Being such a difficult balancing pose it helps to keep you grounded and build focus (dharana). With all of the movements the body goes through we learn to readjust our inner focus.

Source: http://omsweetomyoga.com/posture-of-the-month/ardha-baddha-padmottanasana-standing-half-lotus-bound-forward-bend
http://yoga.isport.com/yoga-guides/how-to-do-half-bound-lotus-standing-forward-fold-in-yoga