Steven Parker207,972 Points
Without the slice, a simple assignment does not make a copy of the list. Instead it would just make the new name reference the same list.
So later, when he sorts the "clean_list", it would also be sorted when you look at it using the name "messy_list".
Besides a slice, you can also make a copy with the built-in method:
clean_list = messy_list.copy()