It's a convenient tool for programming, especially if you program based on percentages and not RPE. Since I'm an excel fanatic, I usually use Brzycki's formula to determine relative intensity for certain percentages (just replace weight with %1RM). And to get even more hify with it, I apply some sort of a percentage for fatigue/set (currently using 1.25% for lower rep ranges) to get some sort of a gauge. It's obviously not 100 % accurate as fatigue depends on rest time, conditioning and other factors, but it points to a direction.
I should probably base the fatigue/set on relative intensity, perhaps also on amount of reps..