a seperate tank that is plumbed with the display tank that usually houses additional substrate, macro algaes and possibly additional live rock.
to increase the water volume of the overall system in an effort to increase stability of water quality. It also provides additional biological filtration surface area by means of the additional substrate/LR
. The macroalgaes are used for nutrient export to help keep nuiance algaes from appearing in the display tank.
thats a matter of opinion.