well the liverock and skimmer (and deep sand bed, if you go that route) IS the filter... good flow is important for (a) passing water around and through your liverock, which contains the various bacteria needed to convert ammonia->nitrite->nitrate->(possibly) nitrogen gas, and (b) preventing waste from settling on the bottom and keeping it in the water column to be removed by the skimmer and liverock. The skimmer of course removes dissolved organics before they completely break down into ammonia. Working together, that is the best filter a saltwater tank can have.
The canister, hang-on back, and even wet-dry sumps w/ filter pads and bioballs end up generating way too much nitrate, which is why they aren't recommended for saltwater tanks.
Sumps are nice, because they (a) provide a way to increase the overall water volume of your system, and (b) provide a convenient, out-of-sight location to house things like your skimmer, carbon/phosban reactor, refugium, UV, etc. The sump isn't NEEDED, but it's certainly nice to have. Its advisable to avoid any kind of mechanical filtration in the sump however, for the same reasons you shouldn't have canisters or hang-on-backs. By mechanical, I mean sponges and filter pads that trap particles... a skimmer is technically "mechanical" filtration too. If you DO decide to include a sponge or filter pad somewhere in your filtration, its highly recommended that you rinse it out daily, to avoid it becoming biologically active and generating nitrates. Since that can become a pain, its easier to just omit any mechanical filtration and rely on good liverock/skimmer/flow filtration + water changes.