Working on a vertical surface will help your child to practice using the proper position of his hand and wrist when drawing and writing. (readingbrightstart.org)
Changing position of hands we displace the load on different muscle groups. (best-powertower.com)
This is because the unnatural positioning of the hands with the palms facie up on a rigid bar puts an excessive pressure on the elbow joint. (fitnessandpower.com)