Physiology of a Kata
Why are Kata's or Martial Arts forms so important, it's simply because they contain symbolic records of human anatomy, physiology and functionality, along with infinite incapacitating actions against an opponents anatomy, physiology and functionality.
Taking the Juji Uke, we can see it could protect us if used as a block because both arms are used for greater bracing and power. However this is just a superficial motion or action, it does not give us the full scope of it's physiological and neurofunctional protection it yields. This is part of the overall Kyusho "Iron Shirt" methods and embedded in many martial forms and styles, which changes your physiology and functionality, in a profound way. It Temporarily negates sensory pain and it's dysfunctional capability from the opponents attack.
The crossing of the hands actually confuses the brain and....
More on the Kyusho's Juji-Uke Protection in the private extended sections below.