What are the common reasons for a barium enema?

Cancer/Infection. Typically, a barium enema is performed to evaluate the colon for cancer, infection, inflammatory diseases. It can be requested when a colonoscopy is incomplete or not able to be performed. See: http://www.Webmd.Com/colorectal-cancer/barium-enema.
Not many. Less sensitive and specific than other tests, like colonoscopy, ct colonography (virtual colonoscopy), barium enemas aren't done much anymore. Can be helpful treating a condition called volvulus (sigmoid colon flips over on itself, causes a usually incomplete bowel obstruction).