When the reader is first introduced to Winnie, she is an unhappy girl. Her parents and grandmother micromanage her, and Winnie feels as if she has little to no freedom. It has gotten to the point where Winnie is considering running away, but her motivations are slightly different from those of most children who might be plotting to run away. She isn't looking for freedom from her house for the sake of freedom. She is looking for the freedom to do something important.  

"Do you know they've hardly ever let me out of this yard all by myself? I'll never be able to do anything important if I stay in here like this. I expect I'd better run away."

The Tuck family gives her just such an opportunity. After Mae strikes and kills the man in the yellow suit, she is taken to jail. Because the man in the yellow suit died, it's hinted that Mae will suffer the gallows. That of course won't work, since Mae is immortal. Her secret would be out. Winnie and the Tucks can't let that happen. Winnie does indeed get her chance to run away and do something important. She sneaks out of her house that night at midnight. Miles breaks open the jail window and pulls Mae out, and Winnie takes her place. Winnie's switch gives the Tuck family time to escape Treegap, because the constable thinks that he still has a prisoner. Winnie's quest to do something is complete, because she has helped save the lives of four people she loves.