size or "volume" of the tank is what decides how you can stock your fish, saltwater or freshwater - the amount of room fish have to "spread" out.
having more water means you have a larger "buffer" against sudden changes in temperature and chemical composition...
the added benefit of a sump allows you to maintain a constant water level in your tank even while doing things like water changes (depending on your plumbing). this helps avoid undue stress on your fish.
sumps are also a great place to 'hide' heaters, filters, excess rock, etc...