I imagine one reason for the minimum check-in time is that it encourages people to turn up early so that everyone, even those who turn up a little late, should be able to get through ontime. To be honest it's a classic trick that is so often used. I've done some coach driving and you always tell people to be back at the coach some time before you actually want to leave, especially if you want to leave ontime (for example getting away before traffic gets busy). I've also found it's good for use of women too. The film starts at/table is booked for 7, you need half an hour to get there, so you tell them you'll pick them up at 6 because you know when you arrive she'll still be getting ready and you'll be just arriving ontime after a manic drive trying to do a 30min journey in about 20mins.