Whether this setting is on or off, the product will still have a chosen option for the attribute when it is put into the cart. So what's the point?
"Required" is used here in the same sense that Drupal's form fields can be "required" (the ones with the red *). If the user does not provide an answer for the field, an error occurs. For products and attributes, the "required" setting makes sure the customer consciously makes a choice rather than simply accepting or overlooking the default option. It's the actions of the customer that are required, rather than the attribute itself.