It is not compulsory to use these if you have a fish only aquarium however they can be beneficial to help hide all of your equipment and they are also beneficial to help the movement of minerals around your tank.  Using a sump can increase the volume of water in your tank in tern making it easier to control your parameters. The smaller the tank the harder it is to control these as any slight + or – can affect the tank dramatically.
Some aquariums may appear to be overstocked but more than likely there is a hidden sump which is doing all the hard work.