I would say that an overflow box is pretty important.
Lets imagine setting up the tank like you say: you create a siphon to pull water out of your tank and then a pump to get it back in from the fuge. What happens when the power goes off/is turned off (like when you do a water change, a feeding, etc.)?
One of two things, depending on how deep the siphon end is in the tank
1) the siphon will keep going, pulling water out of your tank until your fuge overflows.
2) the siphon will break once the water level drops too low and you'll have to completely restart it every time.
What the overflow box allows for is an eternal siphon AND stopping active draining of the tank at just the right water level. That's because the overflow in the tank is piped into a compartment in the back, and this compartment is separated from the drain by a wall. When the power goes off, water will drain out of the tank until the level drops below the teeth of the box in the tank. Then the siphon will equalize, and no more water will flow over the wall and into the drain. During this process, your sump level will rise, but you will have left room in the sump for just this occurance, and the drain will stop before it overflows. So now your tank/sump is automatically balanced.
And when you turn the power back on, the return pump will drain the sump until the DT water rises to the level of the teeth... at which point the siphon, which is still intact, will simply start pulling water and sending it down the drain again.... and now your tank/sump is balanced again.
The box also protects livestock from feeling any hard suction if they get near the box/siphon: the many teeth in the box spread this force over a wide area. This would also protect your box from getting clogged with a snail that crawls into the siphon (and this, by the way, WOULD overflow the tank for sure.
