Quick pressure release vs natural pressure release
The main reason for using the quick release is to prevent overcooking, think about what would happen to white rice if you left the cooker to de-pressurise naturally: it would be mush. Of course, you could factor the time taken to come back to normal pressure into the original cooking time but that's fraught with difficulty because it's dependent on what temperature it is in your kitchen and probably a multitude of other factors we're not aware of.