It's to help make sure that you don't resort to mashing a tall gear at a slow cadence.

You could also tie it with heart rate and speed, trying to figure out whether you need to gear up or down to go faster while not making your heart explode. I tried this out one day on a loop around a small park, with one lap at a higher cadence in a lower gear, and the second lap at a slower cadence in a taller gear. I went faster in the taller gear with the slower cadence, but my heart rate was significantly higher, and I don't think I could've kept it up for much longer.

So, then, If I wanted to get somewhere far away in a hurry, now I know how hard I can push myself while avoiding a major bonk.