It's because cancer cells break off of the original cancer and then spread throughout the body. This causes growths in vital organs like the brain or liver or lungs etc and will cause them to fail as they grow and you die. Usually, doctors will check the lymph nodes closest to the original cancer as lymph nodes filter bacteria/infection and broken off cancer cells as they pass through them. If you have cancer in the lymph nodes closest to the original cancer site then you can tell that some cells have broken off and started to spread throughout the body.

Leukemia is a cancer of the body's blood forming tissue, it causes an overproduction of white blood cells.