There was no guarantee that the war would end when it did. The Armisitice was to negotiate, but negotiations could have broken down. That means that all sides want to ensure they’re in the best possible position when things stop, in case things start back up. So, you can’t simply stop; the enemy might keep taking territory. You need to all agree to a certain time to halt operations. Since communication takes time, you set a time in the near future, particularly a time that is easy to remember and communicate.