When running (and on is pressed) will output a sequence of rendered images of whatever was in your camera view at the current interval time. Useful for outputing a sequential set of images showing the progress of you building your scene. Minimize the script (don't close it) to let it run without cluttering your view. Just hit the off button to turn the timer on after you've setup your other defaults - or hit render now to do a render of the current scene without waiting for the timer.
The only major differences between script packages are slight version compatibility issues - there are no new scripts added between the 3 packages.