Objects in a set do not have an order, so when you display some data on a page, every time a user refreshes the page, the order of the data shown on the page may change, which is kind of annoying. You could use a SortedSet or a JavaScript library (like jQuery Tablesorter used in CSNS2) to sort the data, but sometimes it's just easier to retrieve the data from the database as a list using either @OrderBy or @OrderColumn.