It has been my observation that it is to show any other dog "I'm bigger and more dominant" than you.
Same as when you see a Rooster puff their neck feathers while holding that head high.
It is the sign of a cur or that the dog is intimidated.
It is the sign the dog is amped, geeked-up, and wants to nail something.
It doesn't mean a damned thing, one way or the other.
It has been my observation that it is to show any other dog "I'm bigger and more dominant" than you.
Same as when you see a Rooster puff their neck feathers while holding that head high.