Bound books do not have a flat surface, especially when opened into the middle. When placed under overhead scanner they have curved position and create the same results.
For curved books, the auto flattens option removes the curve from the scanning results and present them as flat documents that are more presentable and easy to read.
Note that not all overhead scanner have this feature. While the higher end models do offer it, when you are going for lower end models this feature may be lacking.