Discograms are done to help diagnose your pain and not to treat your pain. Although dye is injected into your disc and x-ray pictures (and sometimes a CT scan) are taken, the pictures are not as important as your response to the dye being injected (see below).
Discography is most commonly done at the lumbar level for people who have failed extensive conservative treatments yet continue to have disabling low back, buttocks, hip, groin, or thigh pain that may be caused by the disc itself hurting. Often, MRI/CT scans may only show mild degeneration or mild bulging of the disc, and sometimes a tear is seen in the disc. These changes do not necessarily result in a painful disc, unless there is also inflammation. Presently, the only way to determine if the disc itself hurts is by putting dye into the disc. You are sedated and comfortable while the needle is placed into the disc. You are awake and talking while the dye is being injected. With a normal disc, you may feel pressure but no significant pain. However, with a problem disc, one feels considerable pain very comparable to their typical pain pattern. Obviously, this is very helpful in determining the next step in your treatment.