Some spinal conditions aren't visible on X-ray during the early stages. More advanced imaging such as CT scans can detect some conditions very early. Early diagnosis and treatment can make a big difference later on.
For example spondylolysis, a small fracture in one (or both) of the columns of the spinal bone has a better chance of bony healing if found early. Older lesions are less likely to heal completely. Spondylolysis affecting both sides of the column can progress to spondylolisthesis. In this case the bone separates and the main body of the vertebra moves forward over the bone below it.
If and when this happens, pressure is put on the nearby spinal nerves. Pain, weakness, and numbness can occur and interfere with function. As the old saying goes, "An ounce of prevention is worth a pound of cure." Take your doctor's advice. It's a small price to pay for the chance to prevent the problem from getting worse.