Both CT angio and routine coronary angiography have the same goal i.e. visualise the native arteries and the grafts.
If the suspicion of disease requiring intervention is high then invasive angiography is preferred at present because angioplasty if indicated can be done in the same sitting and an angiogram will not have to be repeated.
However if the clinical picture suggests a low probability of significant problem then it is reasonable to have CT angiography. Both the procedures involve injection of dye in any case. Usually an angiography by traditional method will be repeated if CT angio shows significant problem needing intervention.