This video gives a wonderful 'sense' of what the function is of our sensory systems, which is to process incoming auditory, visual, touch, smell and taste information from our environment as well as the two movement senses that processes where we are in space with our body. Then it goes on to show what happens when our sensory systems cannot process properly.
It however doesn't mention the cause of why there can be a Sensory Processing Disorder with a child. Again it is the primitive reflexes that assists in setting up the proper sensory pathways early on in development, from the womb to age 3. By integrating the primitive reflexes through neurodevelopmental movements we can help sensory processing difficulties.