RO/DI is so useful for saltwater is becasue you can start fresh with mixing salt. Tap water varies and some people have more minerals than others so if you just add salt to tap some components of the water will be excess and may lead to something like diatoms/cyano. With RO you can mix salt straight in and get a pretty much perfect blend of salt/tap.
I haven't diddled in the salty side yet, but these are just what I know.
It is completely essential if you plan to keep anything in the tank besides fish. RO/DI should have a TDS count of 0. Most municipal water systems, and most ground water, contains un-knowable amounts of phosphate, calcium, magnesium, and whatever other trace minerals and elements are found in your local water table. The thing people forget is that the ocean is (should be...but that's another story) unchanging. Coral, invertebrates, and all the things that make up the ocean ecosystem are evolved to live in ocean water only. Ocean water is alway the same. The trace elements in ocean water exist in a very delicate balance. You get one thing out of whack and suddenly you have impossible algae outbreaks, cyano drowning your coral, algae altering your pH overnight. The instability that our freshwater fish deal with from rain and runoff and the things that alter their home waters don't happen in the ocean, and the animals that live there cannot handle it like their freshwater cousins.
For that reason, you start with RO/DI and add salt mixes to match ocean water. Can you get away without it? For a while...even I started with just RO water without the DI cartridge, but it ends up causing problems in time.
I kept reef tanks for seven years. I tried all the things everyone does at the start to save $$$... And I learned the hard way just like everyone who as ever tried to reinvent this wheel that nature perfected a billion years ago.
Saltwater fish and inverts are much more sensitive than most fresh water fish. IMO it is essential to have it for saltwater. It removes all the impurities as the oceans are much more stable and constant than fresh water sources are due to the amount of run off in fresh water bodies. Think amazon flooding and not happening in the oceans. Also there are things that could get out of balance if not starting with a pure water source.
if you have a good tap water source that is not too hard IMO you maybe able to get away with a fish only tank. Expect more maitenance and more algae issues. If you have silicates you will have diatom issues. Oh wait what’s that you ask how do you know if these are in your tap water source? Hard to tell because most municipalities don’t test for what aquarium owners do.
If you have well water an RODI is a much and if you plan to keep any inverts it’s a must.
Is it essential? No. Does it take all the guesswork out of your water source and save you a lot of trouble more often than not? ABSOLUTELY. One aspect that folks often don't understand is that the Salt Crystals you add are highly mineralized content. With freshwater you use the minerals and solids that are in treated tap to help maintain a healthy tank, in a saltwater tank with Salt Crystals the minerals and solids in the tapwater source are not only unnecessary but often problematic as they combine with it to create the perfect storm for unwanted algae growth and other issues. Futhermore compare most treated tapwater tanks to RODI tanks and about 90% of the time you can tell when someone has used tap as the water is generally cloudier/murkier in appearance. Can't tell you how many folks I have seen start with tap and after encouragement go to RODI and are shocked by how much clearer/cleaner the tank looks after switching. Bottom line it's already a significant investment and requires a decent amount of maintenance to boot. If you can buy something that is going to make your display look it's best and minimize maintenance and hassle for your investment for as little as the cost of a solid 70-75 gallon HOB Filter, it just makes good sense to do it.
Reverse Osmosis or De-Ionized water is water that has been purified; either by passing through the membrane which does not let suspended solids like salts and minerals through, or by passing though resin beads which remove certain ions from the water. A dechlorinator that removes a few of these is not the same as these purifying systems, which purify the water of all suspended particles. Chlorine, chloramine, and heavy metals in the water is not all that's in the water, and RO/DI do a better job of purifying it; in a surefire way.
And since saltwater creatures like invertebrates and corals are much more sensitive to what's in the water, it's a much better idea to mix your salt with purified water; the leftover minerals that would have come in tap water, would be doubled when salt is added to it. Salt provides those minerals, so you need a pure, pure water to mix it with to have healthy saltwater.