What is a sump for?

[Bristle Worm]

Other than a place to keep equipment out of the display tank

[Poseidon]

More water volume!

It is a place to increase the gallons of your tank! Bigger= More Stable!

[middlemark]

Yup, more water volume.

Beyond being a place for a heater, skimmer, carbon/etc .... and a place to add suppliments, run your auto-top-up into.

[Bristle Worm]

Great thanks..... I forgot that adding supplements to the main tank is not a great idea.

As for water volume that can always be fixed by getting a bigger tank, and therefore for that issue a sump is not critical.

I believe a sump also allows better oxygenation of the water since that as the water falls into the sump in mixes with the ambient air.

Also you can have a reverse lighting cycle to help balance ph and if your sump is quiet enough a place to put additional livestock for whatever reason.

A sump if setup correctly allows for surface skimming of the tank's water as well (this is a big plus). ;;