SwingUtilities.invokeLater() why is it needed?

Swing objects are not thread safe. SwingUtilities.invokeLater() allows a task to be executed at some later point in time, as the name suggests; but more importantly, the task will be executed on the AWT event dispatch thread. When using invokeLater, the task is executed asynchronously; there's also invokeAndWait, which won't return until the task has finished executing.

Some information about the decision not to make Swing thread-safe can be found here: Multithreaded toolkits: A failed dream? [Archived]

Because GUI updates must be done in the event dispatch thread. If you're operating in a different thread, doing the update in invokeLater yanks it out of your thread and into the event thread.

More explanation here: http://www.oracle.com/technetwork/java/painting-140037.html

The smart thing to do with big updates (like repopulating a JTable from the database) on Swing is to get the underlying model, do the updates on the model in your thread, then fire off a notification using invokeLater. That keeps your gui responding to events and redrawing. If the update is going to be very extensive, you can even fire off these notifications with invokeLater at regular intervals while you're updating, like every second or two.