They would have been kinda sunk without her, I think. Lewis (I think) was very knowledgeable about remedies, edibles, etc, but when they were in a pinch, she knew what to do. She translated for them to the Shoshone, one of whom was her long-lost brother, and was able to get the expedition horses for the last part of their journey. Also, their boat capsized and she is credited as saving many of the precious supplies that went overboard. I'm pretty sure William Clark later acted as a father figure to her child (the one that went on the expedition).
That's all I got.