These advanced settings allow you to select which types of vehicles the AI players are allowed to use in game. You can stop the AI using any type of vehicle, e.g. ships or trains, so that if you want to play an entirely water-based-vehicle game, you can.
It is also extremely useful if you are considering taking over many companies during the game and don't want the aircraft that barely scrape a profit because they have to wait for years for a full load.
Sometimes, AIs can get messed up when you close off certain vehicle types to AIs. In PTPAI, for example, if you have trains disabled for AIs, PTPAI will not function and will crash until you re-enable trains for AIs.