I don't understand about Example2 - Analyzing Images in Image Processing Toolbox

To analyze image I'll do follow 12 steps of Example2 but i don't understand about Step 2 Use Morphological Opening to Estimate the Background and Step 4 Subtract the Background Image from the Original Image why did get the background that use Morphological Opening subtract from the Original image?? And how this action have any effect to recieve threshold from graythresh?? and if i don't use step2 why did i get the result not good as use the step2??