Wouldn't it have been easier if we use the hat as our mirror object rather than adding in an empty for the ear muffs?
I would like to know this, too. Can anyone explain why it is or is not better to use the empty?
ssweetbytez and ddigdoug You could do that I don't see why not. But if the hat object origin is not in the center then it will mirror about wherever the hat's object origin is. Same goes for if you move the hat.
Empties are just really handy for that kind of thing and others too. They don't interfere with the render of the scene unlike other objects. I know there are many use cases for them. Part of using it was probably to introduce you to them in the first place and one of their use cases.