They can use it to take a peek at what's happening with your tumor(s), for better or worse, to help assess the efficacy of your treatments, see what you're dealing with at this point in time, and help make decisions on where to go from here.
http://imaging.cancer.gov/patientsandproviders/cancerimaging/ctscans
http://www.cancer.gov/cancertopics/pdq/treatment/pancreatic/Patient/page1/AllPages
Were they giving you Gemzar for all your chemo?
Good luck with your results.