That works for a Word document, but what if you were editing a PHP file? You neither want your changes to be immediately applied while they aren't finished, nor

That works for a Word document, but what if you were editing a PHP file? You neither want your changes to be immediately applied while they aren't finished, nor can the PHP interpreter handle a special format with embedded undo. Or, lets suppose you just log the undo in a hidden file alongside the original. Imagine you've edited a lot of files. All that eternal undo would pile up, and you'd end up with gigabytes wasted on useless undo logs. Plus plenty of stranded logs from moved files. No, the ability to save, to not save yet, and to abandon the undo - those are useful features.