Mist/Wet collectors are used to filter mist that is created by metalworking processes and wet machining (oils and coolants). If not collected, these types of mist can create unsafe working conditions (such as slick floors) and create a situation of high-maintenance (such as when mist builds up on machines or working stations).
There are some metalworking applications that are highly explosive (i.e. titanium dust) in which Mist/Wet collection is highly useful. They are also very useful for high heat applications. The high temperature mist hits a wall of water, cooling it down and bringing it down to be collected.