It increases the total water volume of the system. In SW tanks, the more water volume, the more stable the water parameters will be. It is also good that it hides equipment like a protein
skimmer and heater. It also enables a person to add more
LR to the system and have it hid, so as not to limit swimming space in the display tank. Have you ever had a SW tank before? You will need to start doing a lot of research and reading up on SW, before you really start setting up the tank, if this is your first one.