What is the primary purpose of using geometric constraints in design?

The primary purpose of using geometric constraints in design is to define the relationships between geometric parts. Geometric constraints help ensure that components maintain specific spatial relationships to one another when they are manipulated or modified. For instance, they can keep parts parallel, perpendicular, or at a fixed distance from each other, which is essential in creating a cohesive and functional design. This ability to dictate how objects relate to one another under various conditions allows designers to ensure proper fitting, movement, and functionality of parts within an assembly.

In contrast, the other options do not address the main role of geometric constraints. Adding color pertains to aesthetic aspects rather than structural relationships. Measuring distances is a specific action that might rely on geometric constraints but is not their main purpose. Optimizing weight deals with material selection and design efficiency, which is separate from the role of geometric constraints in defining spatial relationships. Thus, defining relationships is fundamental to the integrity and functionality of the design.