A butterfly’s wings are symmetrical, or a mirror image of each other. Human faces that are more symmetrical are perceived as more beautiful. But symmetry isn’t always the goal of photographers and artists. In fact, they often try to avoid it.
The composition of a photo affects the way the whole picture is perceived. If everything is symmetrical it is balanced and lacks movement. A photographer will use asymmetry (lack of symmetry), or unevenness, to create visual interest, tension and movement.
Imagine two photos of an apple orchard. In one, the trees are perfectly centered in the photo. The composition is symmetrical and lacks interest. In the second, the trees are slightly off center and there is a single fallen apple in the right foreground. The photo is asymmetrical, and the eye is drawn to the apple in the lower right. The composition creates a sense of movement and tension. It's no longer static but tells a story.
Cameras often have built-in viewfinders to frame an image and help select the best composition. Practice with asymmetry by comparing different photos of the same scene to find which composition achieves the feeling you want to convey.