Xanthan gum in bread baking
You seem to have misunderstood something, or been misinformed. 

The only reason that xanthan gum does something in gluten-free bread is that gluten-free bread cannot trap any air for rising and is a very moist and dense thing on its own. The xanthan gum is a kinda OK gluten replacement, imitating its role somewhat. 

It has no such effect in normal dough with gluten. Indeed, it might make it less airy and generally worse, like dough which has too much gluten added. Or it might have so little effect as to be unnoticeable. But it certainly has no mechanism by which it would improve your normal bread dough. 

I cannot say much about the cookies, but it might be that they needed additional binder. This depends on what kind of cookie it was, but some kinds are full of filler and very prone to crumbling, xanthan may help with that. Or somebody backconverted a gluten free recipe to a wheat one and didn't understand that this makes the xanthan obsolete. There are many possible explanations.