It makes sense to take the 5-10 seconds extra to begin a longer process the right way. If the GA had just been lazy and started boarding where the line happened to form, she might have been unnecessarily chastized by a supervisor who might walk by a few minutes later unaware of the wheelchair problem. And your average gate louse, showing up late and seeing boarding taking place through the non-breezeway, would be encouraged to jump in line and crowd the area (as if they need any more encouragement).