When you put a suitcase on top of your car, you have altered (temporarily at least) the structure of the car, affecting its weight, its wind resistance, its gas mileage, its brake distance, its tire wear, etc.

A vertebral subluxation is a change in structure of the spine that affects the function of the nerve system. It causes altered spinal biomechanics, abnormal body function, it drains energy, and causes many other problems.