Juicing is the process of extracting all the liquid from the fiber in a plant. This liquid is where all the vitamins and minerals are stored. When you drink extracted juice your body doesn't have to work at extracting the vitamins and minerals from the plants, you have already done it. This makes it super easy on the body and you get the nutrients you would have if you ate all that food.
Juicing frees up your digestive system so it can focus on detoxing and healing other parts of the body. It is extremely helpful on a fast as well. It helps to keep your energy up and not let your body detox too quickly as a water fast would.
I did juicing for breakfast for about a year staight. It gave me a lot of energy and I think it helped me to get more greens into my diet than I might not have been able to do otherwise. It also helped my mineral deficiency that I found I had. (Before becoming vegan) Now, I mainly eat my greens whole, either through green smoothies or salad. Every now and then I do juicing. I have mainly focused on juicy fruit for breakfast as it is light but also has the sugars I need to jump start my morning.
Juicing is great for those transitioning to this diet from the Standard American Diet because it helps to replenish vitamins and minerals that may have been deficient from their diet.