Wet chemical extinguishers come in multiple varieties, depending on intended use. Class K extinguishers are used on Class A and Class K (kitchen) fires; Water Mist extinguishers use de-ionized water to put out Class A and Class C fires; Foam extinguishers are used on Class A only, or Class A and B only, depending on the model; and Water extinguishers are used on Class A fires only.
Wet chemical extinguishers are good for specific cases, such as in a pool chlorine room, or in an electrical room. Wet chemical extinguishers are used, more often than not, to "fill gaps" that can't be filled using Dry Chemical extinguishers alone.