Unnecessary if you're using a sump and skimmer. You could use it to occasionally polish the water if you like.
Problem long term is that they tend to harbor nitrate. They also do not allow for gas exchange since they are basically a "closed loop".
I never recommend canisters as an exclusive filtration on a marine tank.
They are best suited to freshwater, planted tanks. Helps keep the CO2 higher and pH lower - exactly the opposite of what you are trying to achieve in a marine tank.
Thanks, I knew I heard something about using them on a salt water tank. Ill need to run it till I get a sump that will fit under my stand. I have an acrylic one, I already had the water in the tank before i tried fitting it under the stand.