Surgical oncologists are experts in particular techniques and treatment of various kinds of cancer, whereas general surgeons do not specialize in one particular field.
Almost all surgical oncologists treat all kinds of cancer, but some oncologists specialize during their field of practice and over the years. They provide better treatment for the particular discipline as compared to other surgeons.
It is not necessary always. All kinds of cancer patients can see a surgical oncologist. While common kinds of cancers have better treatments, surgical oncologists also treat rare kinds of cancers.
It depends on the type of tumor. Some tumors are best treated with chemotherapy and radiation while others are successfully treated with surgery.