Are you planning a FO, FOWLR, or reef?
Having a sump is not necessary. There are tanks all over that are run without a sump. A sump increases total water volume, providing greater stability. It's also a great place to hide some equipment -- heaters, protein skimmer, etc.
While a sump increases total water volume, I would not say it greatly increases water quality or decreases the need for water changes. If a tank has water quality problems related to overfeeding, for example, adding a sump would only temporarily make a difference. Within a week or two (depending on the size of the sump added), overfeeding will counteract any effects of simply having more water in the system. Having good water quality has more to do with overall maintenance than it does simply increasing water volume.
The need for a skimmer is heavily debated. I've seen successful reef tanks with and without a skimmer. To an extent, it depends on how heavily you want to stock the tank. A heavily stocked tank will be more difficult to maintain without a skimmer than a lightly stocked tank. Many smaller tanks are run without a skimmer, however, there tends to be more frequent water changes needed to keep high water quality.
Ultimately, I'd forego buying a canister filter, but that's just me. There are better options for biological and mechanical filtration (1 - 1.5 lbs / gallon of LR and a skimmer, for example).