Finding out your pain and activity levels can help the therapist decide what tests to do. The goals and plan of care for you are based on your current symptoms and level of function.
This gives the therapist a baseline for you. The baseline helps measure when progress is being made. Is the treatment working? Are you getting better? There's no sense in continuing treatment when there's no improvement.
Ninety percent of a doctor's or physical therapist's diagnosis is based on information received from the patient in response to questions. The tests and measures used to make the final diagnosis only make up 10 percent of the examination.