From what I've read, MRIs produce better soft tissue images than CT scans, while CT scans give good images of bony structures. In the case of a hemangioma spine, if that's what they suspected, the combination of scans might give them the most information about a blood vessel accumulation in a vertebra. I'm just guessing, however.
Since CT scans involve radiation it's best to limit them to cases where they are needed to make a proper diagnosis or a treatment decision. Presumably that was the case for you. Perhaps they weren't merely looking for a hemangioma but looking for any other spinal problems.
You mentioned in
your other thread that your doctor isn't very communicative. You might have to press harder for answers when you next get a chance.