Depends on stage. Chemotherapy is never used for stage 1 cancers (early). It is sometimes used in stage 2 cancers (early but more advanced than stage 1). Chemotherapy is definitely beneficial in stage 3 cancers (locally advanced) and in stage 4 cancers (distant spread).
No. Chrmotherapy is used for cancers that have spread beyond the wall of the intestine and into the nodes. For tumors that are confined to the wall of the colon it is not usually needed. For tumors that have spread beyond the wall of the intestine but not in nodes, chemo may be used if tumor is high grade, large size causing obstruction or rupture, is close to margin or has high oncotype DX score.