At the lowest end of the scale on flights, the cheapest economy seats, seat selection and boarding time are vital. You need to avid the middle seat at all costs, and you need to get on early to be sure there is enough space for your bag in the overhead.
Moving up in class, seat selection means less. They all have aisle access, and there are few enough people in the cabin stowing your bag is easy. And yet, this is who boards the plane first.