Symmetry makes for stability
We humans perceive symmetry as harmonical. We recognize symmetrical logos more easily, and they signalize a form of stability. This is why we often use symmetry unconsciously in logos.
Symmetry can be utilized in many kinds of ways: One part can be used fourfold - that is, three times at angles of 90° - to create a symmetrical logo. You can also just mirror an element horizontally or vertically, so the elements occur twice.
Sometimes, symmetry is used to create an effect of reflection, e.g. the reflection on water. If a logo bears relation to water, like the logo of an aquaristics specialist shop, its text could be designed to look like it's reflected on a water surface. Properly applied, this is a neat effect that refers to the topic.