Most of what you will study about gestalt is concerned with how these groups are formed and what effect they have on perception. The stronger the grouping, the stronger the gestalt. It is this grouping that contributes to the unity in a design. Gestalt is one of the most powerful tools available to a designer for creating unity. That is the basis for creating variety.
Variety is what adds interest to an image.
The trick is to strike a balance between unity and variety. Understanding gestalt concepts can help a designer control unity and variety. A complex object is really a group of simple items that the mind puts together as a single entity. A face is a collection of eyes, ears, nose, mouth, etc. Your mind supplies the missing parts if enough of the significant features are visible.