If you have a spine, you have a nervous system, which controls every bodily function. If a vertebrae is out of alignment, which is very easily done and often present from birth, it doesn't self-correct or get better on its own, like braces for teeth or a cast for a broken bone, it constantly grows and shifts and needs to be "adjusted". The nervous system controls all bodily function, so if left out of place, can cause pain, numbness, incontinence, headaches, infertility, many other things since all the nerves send all the brain's messages to the body as to how to function properly.
A good chiropractor will take a comprehensive history from you regarding injuries, accidents, posture, lifestyle, activities, etc. Sometimes x-rays if necessary to rule out any degeneration, scoliosis, spinal curvature, etc. The ones I've gone to forever (and my kids too since birth!) don't "make" you come back, but recommend it of course. Most insurances cover enough to go in and correct then to maintain. Mine also adjusts kids for free with the parents. An adjustment can be done manually or with the activator, doesn't hurt, and is very gentle and safe.
I've seen it firsthand improve headaches, arm and hand numbness, hip pain, ear infections, colic, colds, asthma, and many other things. The healthiest people I know go to chiropractors. It's about taking care of problems before they happen, as opposed to just popping a pill to mask symptoms but never alleviate the causes. Not going is like never working out but expecting to stay in shape, or never going to the dentist and just hoping the brushing your own teeth will keep them healthy. Just because you can't see the spine, many people ignore it!
That said, there are quacks of all kinds..MDs included! Prescription drug and surgical errors are in the top causes of death in America, but no one questions those doctors knowledge or judgement? If we would all just take care of our bodies and keep them functional, our nations health would be much better to start with.