Keeping eyes downfield is mandatory, and in this offense, available qb runs during a scramble can be buying time, but also may draw the qb in an open area near the LOS, here a defender may vacate his area, and a qb learns to simply throw over a defender whos dropping down, thats quick blinking while potentially on the run, knowing where you guys are, and keying a certain defensive player while knowing where youre at on the field, and th down marker etc.
A very good qb knows all this, is comfortable, and a rpo is created here, as the qb can throw the ball at any point until hes past the LOS.
This kind of play stresses defenses much more than simply sliding in a pocket, because it maintains the run part of the rpo