They are often used between the fish tank and other forms of filtration to let the solid wastes settle to the bottom so they can be removed from the system. The usually require very regular cleaning to remove those solids before they become anaerobic and start giving off bad smells or gasses. Some form of degass tank should probably be used somewhere after a settling tank just to make sure any harmful gasses are blown out of the water. These things are most often used in raft or NFT systems.
To remove solids. Usually used in conjunction with a DWC or NFT system.
It is less commonly used in media bed / biofilter systems. However, if there is a high density of fish stocked it would make sense to utilize one (and others) to help remove some of the solids since you may be creating more than the system can handle on its own.
There are other options out there though. I personally prefer a swirl filter in conjunction with bio filters. Mostly because it is easy to harvest the fish emulsion for fertilizer in other gardens.
Keep in mind that the fish poo harvested from a swirl filter is not fish emulsion.