Why not kill her, like he did her brother and mother, unless he had some use for her? Likely sexually is what I'm thinking. That's what Ariel Castro did. Kidnapped, chained them up, and had his way. I honestly can't think of any other reason to kill the other 2 and not her. There's a reason she was allowed to live since he had to deal with her on a daily basis (food, toilet needs).
What I'm wondering is how long it took her to chew through her restraints. I can't imagine it was done quickly which leads me to believe that he might not been looking in on her often (so there goes my "check in on her daily" theory. lol). I had braces as a kid and they can be painful just being in your mouth. I had wax to put on some of the projection thingies that hold the wires if I got a mouth sore from them rubbing. And that's without me trying to chew through restraints. Her mouth must have been really painful, yet she did it to get free. But I'd think the pain would slow her efforts down some.
She's a real survivor that one. I wish her the absolute best in life and hope she is able to (somewhat) put this behind her. She's already proved to be strong-willed so maybe she can.