Brassica_rapa or Field Mustard is widely cultivated in uper and lower Punjab in Pakistan and also in India and china
And it is used as vegetable and grass for animals. It is very delicious and yummy. I liked it very much.
In nature, each vegetable has it's own importance but Field Mustard is very very important for health.Each vegetable component is full of health and energy. However, compared to other green vegetables leaf, Field Mustard has it's own importance in all vegetables. 
In ancient times when diseases were treated with the help of herbs, the most doctors used green vegetables and saag (field mustard). But now it is used almost all punjab in pakistan as a vegetable in winter season. 
Field Mustard has Calcium, chlorine, phosphorus, folic acids, proteins, energy, sodium salt Vitamins i.e A, B, C, and many others important salts and minerals that are important for health.
use of Brassica_rapa purifies blood and remove fats from the body. and it clears veins and blood vessels. So guys if you are fat and want to loose your weight then you can use it as vegetable.
At the end I thanks all of you to read this article and I hope all of you happy and healthy
Thanks see you
regards @mabid39