The rule of thirds is a photography principle of composition. It also plays a large role in one of the principles of design: Balance.
If you think about how photographers have taken your portrait over the years, you’ll notice they always put your eyes about 1/3 of the way down the photo. Some photographers do place the top of the head here, which, in my opinion, is less interesting… who cares what the top of your head and the background above it look like! We want to see your face!