Viparita Karani mudra is known as Viparita Karani asana also. It is similar to the sarvanga asana which means asana in all the organs.
The best benefits are
Blood Flow: As we see the posture is against the magnetic field for the earth to heart can easily pump the blood to the head.
Organs benefits: As the blood can flow naturally and get the help of the margaric field so the organs get good / sufficient amount of blood. so that proper oxygen and nutrition helps to work more efficiently.
Easy blood flow to the brain: As blood reaches to the brain easily so the brains cells get more energy and feel calm and peaceful.
Benefits for Thyroid gland: We know Thyroid gland plays a great role for our emotional stabilities/instabilities regulate the mental and physical changes from the birth till death so here a contraction over thyroid gland and adequate blood flow helps to work more efficiently in our body.
Hair Growth: Our skull and related organs needs more blood / nutrition for better hair growth. and here it plays a positive role for hair growth.
Except all these it is benefitable for digestion, sleep, and other conditions like restless leg syndrome, fibromyalgia, menopause, and metabolic syndrome.