If the cancer is essentially incurable but is causing problems that can be remedied then treatment for relief of symptoms is indicated. In this situation quality of life and not longevity is the goal. Treatment is sometimes given even when all the cancer is thought to have been removed. If there is a significant statistical risk that the cancer will reappear later, then prophylactic treatment is given. This is common in breast cancer.
Cancers cause problems because they grow and occupy space.
They push on other structures and cause them to malfunction. They erode into other structures and bleed. Cancers have no nerves so they do not hurt. They can cause other structures to hurt because of the destruction or pressure on the normal structures. Cancers require energy to grow and rob the body of the energy it needs for normal function.