Stretching the skin makes for a much smoother shaving action. With the skin taught, the blade meets the hairs and the hairs move very little allowing them to be easily cut. When teh skin is not tight, then when the blade meets the hairs, the hair moves with the elasticity of the skin until the pressure from the blade overcomes the resistance from the hair and then the cutting happens.

I've found that WTG and XTG stretching isn't nearly as vital as when shaving ATG.