We report qualitative findings from interviews and observations detailing how professionals generate and evaluate design ideas using design optimization tools. We interviewed 18 architects and manufacturing design professionals. We frame our findings using the Geneplore model of creative cognition and classify examples of ideation and abstract design thinking arising from optimization workflows. Contrary to our expectations, we found that the computed optimum was often used as the starting point for design exploration, not the end product. We also found that parametric models, plus their associated parameters and simulations, serve as an alternate, highly valued form of design documentation distinct from engineering schematics.