Cancerous tumors are removed so that they do not metastasize. Some cancers are even curable if they are caught early. At other times both cancerous and benign tumors will be removed if they interfere with the function of an organ in some way. Sometimes a tumor may be removed for the purposes of a biopsy. Physicians may not always remove tumors. In many cases of metastatic cancer, the removal of a tumor is of no medical benefit.
Hope this helps. :D