Originally Posted by Tino
The main reason is board early in the "A" group is that you will need to rest your feet after standing on them for an extra half hour.
If you look at this from the other angle, you Stand in like because you know that you will be SITTING for the next hour, two hours, however long your flight is, so you don't want to be sitting in the chairs at the gate until the last minute. It works both ways.