If a tooth is causing pain, a root canal can often resolve the pain.
Sometimes a tooth may need a root canal even if there is no pain. This happens when there is infection present (chronic apical abscess) or when infection will soon develop (necrotic tooth). If left untreated, this infection can spread to other teeth or become life-threatening.
See this example:
Tooth #10 has no cavity but the nerve died. Trauma was likely the cause. The infection grew and is now almost affecting the adjacent teeth.