Having a closed off gate area where people can be boarded as they enter, then sit and wait is a huge help to the airlines as it allows passengers to be processed whilst cleaning/catering etc is going on, thus any missing passengers can be found earlier.
Ah, I can see you've never been to London City Airport then, where this is the normal mode of operation.
It's an absolute bloody nightmare being herded into a little "pre-board" room
Leave us in the lounge until you're ready for us to enter the plane!