There is no guarantee that any security staff understand the reasons they do anything. I think the reason it had to seal was to stop people stuffing so much it was all falling out of the bag and making a nonsense of the 1L limit on volume for the bag. The vapour reason is becoming the common answer but in 2006 at our check point we had no vapour type checking machines available. As for 'keeping it all together' why does that matter. As the years march on the real reasons for the whole circus are forgotten and myths are repeated so often that they become the new reasons.