It's similar to the shot clock in basketball prior to the end of the half. If you time your possession correctly (which is of course secondary to scoring), then you get points and the extra possession. I agree in football often times it isn't possible, but it's not completely due to happening. Yes, the opportunity is clearer in basketball (especially since you can more control 2 for 1s), but the same general concept still applies in football which is don't leave time on the clock for your opponent to score.