The update method is used to bring the entire application world ``up to date.'' It flushes all pending output to the display, waits for the server to process that output and return errors or events, handles all pending events of any sort (including when-idle handlers), and repeats this set of operations until there are no pending events, no pending when-idle handlers, no pending output to the server, and no operations still outstanding at the server.
The idletasks method behaves as above but no new events or errors are processed; only when-idle idlers are invoked. This causes operations that are normally deferred, such as display updates and window layout calculations, to be performed immediately.
The idletasks method is useful in scripts where changes have been made to the application's state and you want those changes to appear on the display immediately, rather than waiting for the script to complete. Most display updates are performed as idle handlers, so idletasks will cause them to run. However, there are some kinds of updates that only happen in response to events, such as those triggered by window size changes; these updates will not occur in idletasks.
The update method is useful in scripts where you are performing a long-running computation but you still want the application to respond to user interactions; if you occasionally call update then user input will be processed during the next call to update.