A root canal (or endodontic treatment) is indicated in cases where a tooth's nerve has died or is dying and the root canal space has become infected with bacteria. The problem is, not all nerve death is accompanied by symptoms, and studies show that up to 60% of nerves can die without the patient even realising it. The causes of nerve death are variable but the end result is the nerve space becomes infected with bacteria. These bacteria proliferate and track down the root canal until they reach the jaw bone which then also becomes infected.
This may all take place sub-clinically i.e. that is without any overt pain or symptoms from the tooth to alert the patient to the problem. These conditions are usually picked up at a check-up visit, where the dentist might notice that the offending tooth is slightly darker in shade than it's neighbours and will follow this up with a vitality test and a periapical x-ray. In advanced cases the x-ray will a dark circular area around the root tip/s indicating an inflammatory response and infection of the bone.
These lesions can remain dormant for an indefinite period, but eventually they cause problems by producing acute pain and swelling of the surrounding tissues. Once discovered the treatment of choice is to carry out a root canal. If the patient does not wish to have a root canal an extraction is the only other available option. Leaving it would not solve the issue, and only makes it more difficult to treat in the long term and obviously there is the risk of severe pain should the lesion 'blow'.