Okay, Cicero, here's another example.
You did the whole calculation* and it says you're about to hit a 189 yard shot. With your 4-iron (210Y), it'd be 90% if the meter was linear but we know it's not. According to the chart, looking at 212.5 Y, it should be tad more than 90%, almost 91%. I know, I know... It's not much, not a dramatic difference, and for a 189 yard shot, you'd rather go with 5I. This is just to illustrate an example. (* Whether your calculation method is accurate or not, it's another story though.)
Once again, this is just MY interpretation of the chart so maybe there's a better way of explaining it. The best is the author comes in and explains it himself. ;)