What are practical advantages and disadvantages of learning to use both hands to draw? This question is not only about using both hands simultaneously, it's about being able to use either of your hands in general. By practical, I mean advantages and disadvantages in terms of drawing. In other words I'm not talking about cut of aggression level etc. 

I've started learning to draw recently using Draw a Box lessons and wondering whether I should do the offered exercises with both hands or just focus on my dominant hand. I read somewhere that some professional painters use their non-dominant hand for less skill demanding tasks, so their dominant hand gets less worn out or tired. Others say that they learn to use their non-dominant hand so they can use it for work in case they injure their dominant one.
I meet with a small group once a month to "do art". Each month one of us is in charge of the lesson. We just did pencil sketches using our dominant and then non dominant hand. We were all amazed- everyone liked the non dominant drawings better. You are more detail oriented and try to draw to perfection w your dominant hand. The non dominant  drawings were looser and more free. It is considered a good practice to do to increase your awareness to detail, your drawing skills and good for your brain.