1.) Because of a vitamin deficiency, the poop offers something that they are not getting from their daily meals 2.) This behavior could be an instinct passed on from before dog domestication - mother dogs would eat their pup's feces to cover their trail from predators3.) Or it could be that they just like the taste of it.
While this is a natural behavior in dogs, understandably humans find the behavior quite repulsive. Obviously the easiest way to stop a dog from eating her own faeces is to remove the poo before she has chance to eat it as you have been. If your girl is an indoor dog, you can use that to your advantage. When she goes to the door to be let outside, quickly run outside to clean up any stools that might be in the yard, or follow her outside and if she poops, clean it up before she gets a chance to eat it! This will be difficult to do whilst you are away at work however and so there are several other tricks you can try.
When you are home you could try lacing some of your girl's stools with pepper or bitter apple or perhaps you could try Tabasco sauce, or lemon juice. However, some dogs will eat the poop regardless of the taste. There is a product on the market called Forbid that is added to a dog's diet to make feces distasteful, though this is not always effective - you could definitely give it a go with your girl! Some people have added pineapple or spinach to their dogs diet, while others use a meat tenderizer. This makes the poop less palatable. You could also try to teach your girl not to touch her feces by issuing a reprimand when she goes to eat it. Leave a stool in your yard, then when she begins to eat it, squirt her with cold water or make a loud noise (such as shaking a can of pebbles) and growl loudly at her. If you do this consistently, she will soon come to associate the two, which will hopefully put her off eating any more faeces.