It's not a stupid question. It's a matter of stages of engagement. The face and eyes indicate intention. You can't successfully track or kill without focusing

It's not a stupid question. It's a matter of stages of engagement. The face and eyes indicate intention. You can't successfully track or kill without focusing your full attention on the object, and the easiest way to detect this is by seeing where the eyes are focused. Once a threat is determined, then focus moves to other body parts.