dehumidifier

Has anyone used a car DEHUMIDIFIER bag for their car, or are they any good..

My W124 has got 3 buttons in the centre of the heater controls:- Top is the dehumidifier Middle is the air-con (snowflake, ie default is a/c off) Bottom is re-circulate What is the difference between dehumidify and air-con ? Surely air-con will dehumidify the air so why seperate buttons...