Symmetry can be considered as predictable and perfect regularity within pattern. In symmetric pattern, certain aspect(s) of the pattern are produced identically when other aspects of the pattern are changed.
In other words, symmetry is a fundamental part of geometry, nature and shapes. It creates patterns that help us organize our world conceptually. We see symmetry every day, but often don't (consciously) realize it.
It is actually an important and prominent feature of the visual world. It has been studied as a basis for image segmentation and perceptual organisation, but it also plays a role in higher level processes, such as face and object perception.