For the ducts, the setting indicates where you should set the switch based on the season. If your house has separate air return ducts on different floors or even multiple ducts on the same floor, changing the setting changes which ducts pull air from the house, allowing lower ducts to suck cold air out in the winter and higher ducts to suck hot air out in the summer.
This matters because air-flow more easily goes to where there's a space for it - so pulling OUT the hot air, which rises, in the summer means the cold air has room to move up, and the warm air that got pulled out of the room gets cooled. If you had low returns open in the summer, the cold air that comes in the room and settles to the floor gets sucked out again and the hot air that rises stays up top.
The humidifier settings, however, are likely to increase comfort based on the season. You want more humidification in the winter (which has drier air) and less in the summer cause hot muggy air sucks worse than hot air.