If Dumbledore can lift the Anti-Apparition wards even temporarily, like in the Great Hall, why didn't he do it in the Astronomy Tower? Wouldn't that have allowed him to do more in the Battle of Astronomy of Tower? If he couldn't Apparate because he was weak, Harry could have Apparated him as he had done from the Crystal Cave to Hogsmeade.
As you may know, it is usually impossible to Apparate or Disapparate within Hogwarts. The headmaster has lifted this enchantment, purely within the Great Hall, for one hour, so as to enable you to practice. May I emphasize that you will not be able to Apparate outside the walls of this Hall, and that you would be unwise to try.