Although a sump or a refugium is not a requirement for any saltwater tank, there are many reasons why they are so popular. To start with they add a lot of volume, which helps a lot with chemical stability. They also offer somewhere to hide a lot of equipment. Saltwater tanks can be a lot more complete and balanced as a whole than freshwater, IF you rely on natural filtration. As stated the live rock is most of your filtration, but the other two most valuable forms are a protein skimmer and a refugium. In addition, a sump uses an overflow which helps keep the surface of the tank clean, which is a problem in many tanks.
HOBs, canisters, etc. are all filters made for freshwater tanks. When used on marine tanks they usually do little more than trap a bunch of debris that will sit and rot, lowering the water quality unless you clean the filter every single month without exception, ever.
It can be done, but a system that is setup the best will run the best, and I wouldn't consider standard HOBs and canisters as doing that. They will create a lot of unnecessary maintenance.