While steam and drum whole-home humidifiers aren't used as often as whole home humidifiers, there are some very specific uses for them.
Steam humidifiers mount under a supply or return air duct. A heating element boils water in a reservoir, and steam blows through the ductwork when needed. Steam humidifiers can come with or without flushing systems to exchange water. They serve very specific needs and aren't for most consumers.
Drum humidifiers , like flow-through, rely on the normal pressure-differential created in a forced-air heating system to direct a portion of the air to the humidifier. The pad is mounted to a drum, which rotates through a reservoir of water. They can come with or without a flushing timer, so they are good for areas without access to a drain and can be a lot easier on water than steam humidifiers.
Drum humidifiers can save a lot of water and a lot of money on operating costs, so they're good if that is important to the homeowner. Steam humidifiers cost a lot more but are good for large buildings and places where consistently regulated humidity is needed.