We don't know a lot of her backstory but that doesn't mean I cannot prove this theory.
Emerald has done some bad things in her life. This includes stealing,killing, and blackmail. However in her position you would do the same thing.
We know Emerald is from Vacuo because of the city that Cinder confronts her in. Because of its desert sandy background it is safe to assume this as in the board game team RWBY played and when Sun stated as "Hey, I may have moved to Mistral, but I grew up in Vacuo. It's not exactly a shirt-and-tie kind of place."
We don't know who her parents are or whether they are even alive or are not abusive. All we know is that she went hungry a lot this can be made a conclusion when Cinder tells her you will never go hungry again.
The thank you that Emerald gives Cinder is a bit saddened and grateful. Like someone gets when they are very depressed and starving.
So she did it to be able to live a life worth living not worried about if she will be able to feed herself or be able to live another day.
I believe she does what she does so that she won't have to return. Just following Cinders orders to be able to live another day.