Fuselage AoA in cruise
Not entirely sure, but I guess the designer optimises the wing incidence angle based on several considerations, one of which should be good fuel economy in cruise, or low drag. So the 2 to 3 deg fuselage AoA of the B737 in cruise probably minimises overall drag by aligning the fuselage to the overall airflow direction; which should be a little downwards due to the action of the wings.