Passengers are boarded by the seat row to speed up the boarding process. Its done to stop the delays where everyone has to stand in the isles to put their hand baggage in the overhead bins. Now if it starts from the rear then there is less chance of boarding being held up by a passenger in a forward row holding people up whilst they get them selfs sorted. Unfortunaly some passengers don't think this applies to them and try and get through the gate. Gate staff usually tell them to move to one side and listen to the announcments but there is only sop much they can do.
Back to the holding area, Passengers are usually called to the gate lounge 10-15 mins before the aircraft is ready for boarding. This is to make sure that most of the passengers are there and not farting around in other areas of the airport. But of course some passengers turn deaf and dont hear the calls to the gate or turn blind and cannot read the boarding times on their boarding pass or info screens.