To clarify, you are asking how subjective assessment, not objective assessment affects program design.
Subjective assessments including general health history, medical history and lifestyle helps me to understand my clients personal history and how things like their occupation and recreational activities/hobbies might be affecting them. The subjective assessment is just as critical to the development of a training plan as the objective assessments (postural analysis, cardio assessment, body comp etc…).
Subjective assessments give me clues as to the structure and function of my new client. I obtain information about their capacity for movement as well as any repetitive movement patterns that could affect muscular imbalances. By learning about their hobbies and recreational activities I can assess for proper forms of training to increase the efficiency of the kinetic chain and avoid injury. By learning about past injuries and surgeries I get clues as to possible kinetic chain dysfunctions. And by knowing what medications they might be taking I become aware of the side-effects they may be experiencing.