Sometimes the plane is there but not the crew, and in these cases getting the bags on would be a real time saver.
Also, it would avoid the situation where inconsiderate jerks put 2 bags in the overhead. One could also ensure that rollaboards went on the 3-seat side, other bags on the 2-seat side (where appropriate), and that people in the back didn't dump their bags in the front on the way back.
The only downside is for STE+ who arrive too late for valet and no longer get any benefit from early boarding to secure bin space.