Symmetry is a widely taught compositional technique used for creating visual balance in your photos. We unconsciously look for symmetry in all things, and a photograph with elements that are almost perfectly balanced usually makes for a very appealing image.
Just take the photo of the Eiffel Tower above. It may have been incredibly easy for the photographer to position himself and the camera in a way that will capture the silhouetted French landmark, the dramatic sunset, and the mirror image at the bottom of the frame, but a viewer would take longer than usual to observe the many details of this photo, thus making it all the more intriguing.
When executed properly, even a visually imbalanced image can turn out to be an interesting photo. As much as we find pleasure in looking at anything symmetrical, there’s something unsettling yet intriguing about an image that doesn’t seem to “follow the rules.” This informal balance technique is a lot more difficult to achieve, but it gets better with practice.