If the idletasks option is specified as an argument to the procedure, then 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 update idletasks procedure 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 update 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 update idletasks.
The update procedure with no options 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.