cystoscopy after bladder cancer

hi doctor,
I am a bladder cancer patient, and I have received cystectomy for quite sometime , (removal of whole bladder and my left kidney), now everything seem to be ok with exception of difficult urination. at the present moment , I still have to undergo checking every now and then. I just want to know as I have receive ct scan of my whole urinary tract (from kidney to the urine outlet, then why my doctor still want me to have cystoscopy done (bladder checking)?
thank you for your advice
taiposek