USG, CT scan or MRI are anatomical imaging modalities. They give information about size, location, relations of internal organs and contrast enhancement pattern. However, they do not give much information about metabolism or functional status of the organs in general. PET scan gives metabolic information not available on CT scan or MRI. This is very useful for diagnosis and treatment monitoring. As metabolic changes happen before anatomical changes, possibility of early detection of disease or detection of changes after therapy is greater with PET scan.
Further usually USG, CT scan or MRI is done for particular region of the body. While PET CT scan is generally whole body scan when done in cancer patients. It helps in detection of disease spread in the entire body in single examination.