Technically, it doesn't make much sense. The take-off time of a rocket is called T0. It is estimated that after watching so many rocket launches, few people really heard what No.0 (the one who pressed the launch button) said after the launch (that is, T0, XX minutes, XX seconds, xxx milliseconds), at least I didn't hear "T0" clearly before. Time T0 is of course important for orbit calculation. The problem is that the current automatic launch management system will automatically transmit the time T0 to all monitoring stations as soon as the rocket is launched. Therefore, whether to read time T0 depends mainly on the live and TV viewers. It doesn't matter, even if it is earlier or later. Because the general low-earth orbit spacecraft does not have high requirements for the launch window, even deep space probes, such as Chang 'e spacecraft, do not have to launch in a certain second although the launch window is short. However, the invention of countdown itself is not due to technical considerations, but mainly through countdown, which gives people a sense of urgency of time and has a great psychological effect on all people involved in the launch. When a rocket is launched, there are so many people who need to be mobilized, from the survey ship in the Pacific Ocean to the people who finally leave the launcher, which definitely needs unified command and dispatch. It is very useful to listen to the countdown and keep up with the pace of work of all participants. Because although most of the work is done automatically, it still needs people to trigger it. Pressing the launch button is just a link, and there are many other kinds of work that require people to give initial instructions. Oral countdown is always for people, and rockets don't listen to countdown. American rockets are all ignited in countdown and lifted off directly at zero.