It's where you put your tongue to keep your breathing unobstructed. In Breath: The New Science of a Lost Art, the author James Nestor discusses at length the how the jaw and mouth size of modern humans has decreased due to the invention of cooking and how the this smaller aperture gets obstructed. In reference to this specific point:
He [a dentist] explained that the first step to improving airway obstruction wasn't orthodontics but instead involved maintaining "correct oral posture." Anyone could to this, and it was free.
It just meant holding the lips together, teeth lightly touching, with your tongue on the roof of the mouth. Hold the head up perpendicular to the body and don't kink the neck...
It should be straightforward to compare other tongue positions. You should find that other positions like tucking the tip into your lower jaw, for example, result in more tension, restricted breathing, or noisier breathing.
I am not aware of any specific link between the tongue position at the roof of the mouth behind the teeth and fighting; if someone gives this advice it's also the position they will suggest for resting.