Correct.
Correct. Altitude based on just baro data can drift over time, e.g. when the atmospheric pressure changes (aka weather) or the pressure changes based on flight direction, temperature, etc. So, GPS altitude can help to correct the altitude estimate over time, plus it will allow you to have the “absolute” altitude and not just a relative altitude relative to when you powered the Pixhawk on (or set the pressure at sea level manually).
Depends on what “not very good” means. If you are for instance flying indoor, then it might indeed be advisable not to use the GPS because it will be noisy, especially in altitude.
You could also consider using a distance sensor pointing down to improve altitude tracking.