Many light general aviation aircraft-- and perhaps many other aircraft as well-- are designed with some of amount of downthrust in the engine mounting, relative to the centerline of the fuselage. Presumably the wing incidence angle is chosen to streamline the fuselage in some particular flight regime. So why is any downthrust designed into the engine mount? Exactly what is the designer trying to optimize, by including downthrust in the design? In what undesirable way would the aircraft's flight characteristics be different if the downthrust were not there?
For example, is the intention that adding power (without re-trimming the elevator) should not make the aircraft climb? Or that adding power should not cause a change in trim airspeed? Or that adding power should not cause a change in trim angle-of-attack? Would there be a "wrong-way" change in trim airspeed when power was added, if the downthrust were not present? If so, why?
Note that one reason for some reduction in trim airspeed when power is added to climb but angle-of-attack is held constant, is given in this related answer. (See the tables-- and note that a similar reduction in trim airspeed would also occur when power was reduced to descend.) The present question anticipates that this effect may be minor enough not to be the main reason for including downthrust in the design of any particular airplane.
This question is aimed primarily at aircraft with nose-mounted propellers and conventional tailed configuration, so please be sure to address this configuration, but feel free to discuss other configurations as well.