Consider the following:
210x8,8,8,8 = a total of 6720 lbs over four sets
210x8 and 215x7,6,6 = a total of 5765 lbs over four sets.
Moreover, I think you have to think of this in terms of cumulative results over multiple workouts to see that the double progression model gives your body time to adapt to the stress and grow before attempting to increase the weight.
Always trying to increase the weights as soon as possible leads to quickly burning out and stalling, in my experience.