Rule of thirds
See the picture below.
The eyes, the mouth, and the center of the face are all on one-third lines perfectly.
Let's take the lines away.
Now the picture has a good sense arranging a balanced composition.
If placing the face into the middle, the picture becomes boring, line a photo taken by an amateur photographer.
The rule of thirds may not only for photography, but also for everything. Try this rule when you take a photo next time.