Trivia: As a young boy and teen when Apollo and Space Shuttle were in their hey days, I often wondered why, in every launch, they seemed to make a specific comment that, to me, seemed superfluous. But it was always there. For a long time, I thought it was just tradition or ritual, but it wasn't.

Turns out that there are two sets of controllers involved in the mission. You have Launch Control, at the Cape, and you have Mission Control, in Houston. There needs to be a specific handoff from one to the other so that there is always exactly one set of controllers in control of the flight. A pretty obvious demarcation would be to have Launch Control get the vehicle into orbit and then turn it over to Mission Control. But that's a mushy milestone. When, exactly, is the moment that the vehicle goes from not being in orbit to being in orbit?

So a very clear and distinct event was chosen as the hand-off point and a verbal declaration is made by Launch Control to signal the handoff while Mission Control knows exactly when it is going to happen down to a fraction of a second and is ready to immediately accept it. The magic words are, "... has cleared the tower."