YOUR FUEL GAUGE
One of my less than proud moments was when I ran my vehicle out of gas. I've done it twice. The first time was completely on me. I was in college and thought I could push it a bit longer. The second time was kinda on me. I was on my motorcycle but it didn't have a fuel gauge. I miscalculated on the mileage and ended up high and dry.
Our bodies work the same way.
Think of your food log as the fuel gauge on your car. While you 'fill it up' you know how much energy your body has to use. Certainly like your car, you don't want to over fill it, but if you underfill, you might find you run out of gas at some point too.
I am a creature of habit and even though I eat basically the same thing each day, I still catch myself not hitting it 100% if I don't log.
Give yourself the advantage of knowing what your internal gas tank has. Whether you overfill and 'spill over' or are too low and run out of gas, either scenario is less than awesome.