Hi Jeremy !
Welcome to the board !!
Actually that's a pretty good question.
As LemonShark mentioned, you'll almost always find some sort of sump or container underneath the tank in the stand.
Imagine for a second that you are pumping saltwater into your tank from a large trashcan of saltwater.
Eventually the tankwater level would keep rising and would overflow onto the floor - right.
An overflow allows this excess water that is being pumped into the tank to "over-flow" in a controlled manner, and not spill over the sides of the tank onto the floor. As this saltwater flows out of the tank it normally falls down through a pipe or drainhose to a sump or wet/dry filter below the tank. This sump can be a smaller glass tank, or rubbermaid plastic container, or one of several other designs.
In the sump is the pump ( just like the one described in the trashcan above ).
This pump is called a "return" pump .... because it returns saltwater in the sump back up to the tank.
It returns the water that came down the overflow.
An overflow can either be drilled tank hole that allows the water to fall down to the sump, or the overflow can be an external hang on the back type. The external ones have a U tube that allows the water in the tank to rise up and over the top edge of the tank, and then this water proceeds to fall down to the sump.
The biggest advantage I see when using an overflow .. you allow the very top layer of tankwater to be skimmed off the surface, and then this water can make it's way to the sump.
Down in the sump, many people will either have additional biofiltration like bioballs, or filter media.
Or they may have an in-sump protein skimmer.
Or they may have a planted macroalgae refugium.
Or they may grow pods in the refugium.
Or they may place some activated carbon down there.
Or they dose kalkwasser there.
Or they may dose other additives there.
Or they will do their water chages there.
Or they will place their heater down there.
Or they will run a calcium reactor there.
Or they will do any or all combinations of the above.
The real advantage of having an overflow ... is that it allows you to have a sump. Once you have the sump ........ your choices of how you want to filter, dose and maintain your water conditions increase greatly.
The overflow without the sump is meaningless ...... but a sump with a return pump, and a hose or pipe running from this pump back up to the tank will push water into the tank.
As the level in the tank rises ... it will naturally overflow back down another hose or pipe to the sump.
The water just goes around and around and around all the time the pump is running. What goes up must come down.
Since the water is always circulating like this ( as long as the pump's on ) you can place a lot of your equipment in the sump.
Heaters, skimmers, dosing equipment and the list goes on and on.
Overflows and sumps are a wonderful combination to have .... not absolutely necessary by no means ..... but they do give you a lot of flexibility and options to choose from - and most often allow you to keep the display tank clear of electrical equipment.
If I went into too much detail - sorry - drank a lot of coffee this morning