justhere

If the flight is ready to go, why sit not moving if you can get going? Why have the plane sit for 10 minutes just because? In order to depart the gate there are myriad of things that have to happen. If the departure time is, say, 5PM, they can't make sure that everyone is seated, all check lists complete, all doors closed, jet-bridge or jet-stairs removed, etc, etc, at 4:59PM and hope that it all only takes 60 seconds to complete. So they, airlines, have parameters based on plane size along with a bunch of other important stuff that has to be done before pushback Those parameters are almost certainly based on standard practices and established time-studies.

So if you need to pushback at 5PM, you work backwards to establish what time the door closes, what time boarding needs to be complete, etc. If, through a series of events, some of those things don't take as long as normal, you may not know that's going to happen until it happens. But you've started them on time, you just happen to finish them early. But when you've finished them, you've finished them and you're ready to go, even if that is 10 minutes early.

If you then say "let's leave the boarding door open in case any late connections can make it on by 4:51, or 4:55. or 4:59:30, then you likely have to start some of the preflight stuff over again or at least take the time to update stuff. And before you know it, it's now 5:02 or 5:08, etc, and you are late and possibly putting your flight's slot at risk.