Giving the user the ability to change the sorting order of a table has powerful effects. First, it facilitates exploration. A user can now learn things from the data that he may never have seen otherwise (how many of this kind? what proportion of this to that? is there only one of these? what's first or last?) Suddenly finding specific items becomes easier, too; a user need only remember one attribute of the item in question (e.g. its last-edited date), sort on that attribute, and look up the value he remembers.
