You don't need a sump but they sure make life easier and make for a better looking tank. They help by increasing total system volume which is a definite plus for SW
. Plus, they get all the equipment out of/off of the main tank and put them out of sight. You're left with powerheads and return plumbing in the main tank. Looks much nicer than heaters, thermometers, filters, skimmers etc.
If you are planning a fish only tank, with no possibility of live rock and corals, then you would be fine wilth a canister filter. If you like the reef tanks, (and have the money...it gets expensive) then your live rock will do your bio-filtration for you plus be the aquascape to build your reef off of.
Canister filters can be hazardous to use for reef tanks. They get to be nitrate factories and need CONSTANT maintenance for a reef application. Not recommended.
Water changes can be done monthly, but many of us do them bi-weekly still.
P.S. A good skimmer goes a long way in SW