Your car rolls on bearings. They are lubricated with grease or oil and that lubricant is held in place by seals. Brake Masters recommends that your bearings and seals be re-lubed or replaced every time you perform a brake job or 36 months whichever occurs first.
Some cars have sealed bearings, they do not need periodic maintenance, but they do need to be inspected and at times replaced. Bad bearings can cause excessive tire wear and can even lead to break downs that can be potentially dangerous.