Cure diseases. A bmt or stem cell transplant replaces a patient's bone marrow with that of someone else (allogeneic transplant) or by a patient's own marrow as a rescue from very high dose chemotherapy. With allogeneic transplant for cancers, the new bone marrow can attack and destroy cancer cells that it recognizes as bad, even though the patient's immune system failed to do that.
Treatment. The point of a bone marrow transplant (there is no "blood" transplant) is to allow your bone marrow to produce red and white blood cells. This is typically done for a disease of the bone marrow that has resulted in marrow failure or a hematologic malignancy that requires treatment that essentially wipes out the bone marrow. Your hematologist should be able to tell you more.