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.
Prognosis depends on many factors. The prognosis for any one individual cannot be predicted, but if there were a large number of people all in the same situation then probabilities of cure, remission, survival and death can be predicted. Some cancers are very predictable others are not. Some cancers that would be expected to be easily cured are not. Some people live with their incurable untreatable cancers for years.