It is known that every cell in your body is controlled by your nerve system. It is also known that stress on the spinal joints or structures results in interference in the functioning of the spinal nerves. Thus when your spine is subluxated the nerves will suffer and ultimately the organs or tissues that the nerves supply will also suffer.
Chiropractors do not consider a lack of pain to be a key marker in your state of health. Chiropractors look at how well you function as an indicator of your state of health. For instance top athletes often don't have pain, but demand more from their body, hence they seek Chiropractic care to ensure their spine and nerve system are working optimally.
Many of us are completely unaware of the millions of processes occurring in our body every day. For instance most people are not aware of their heart beating or their stomach working - your body takes care of all that for you. Thus wouldn't you want to make sure the communication lines from your organs to your brain are working at 100%?