The benefit is starting at "ground zero" ... almost 0 ppm. This gives the grower the ability to have COMPLETE control over the nutrients the plants uptake. The biggest problem in using one is having false expectations of what it will accomplish ... for example, you're starting with an alkaline water source that pH's at 9.0 before filtering ... all minerals and suspended solids are removed down to "essentially" zero parts per million ... Why is my pH still testing at 9.0?
The answer is kind of complicated and involves understanding water chemistry. In this case, you still need to buffer the water or you can see some wild pH swings. I have one ... and I disconnected the RO cartridge (they are expensive) and I'm only using a 2 stage filtration to remove sediments and chlorine. This is plenty enough for a soil grower.
RO also is very wasteful. For every gallon of RO water you claim, you're wasting up to a gallon down the drain. If your water is metered and you've got a large grow (yours is small enough it wouldn't make a big difference) then you are literally pouring dollar bills down the drain. RO would make sense in a commercial establishment. The run-off from fertigation is claimed in a collection tank and recycled back through an RO system so you're back at the starting point of essentially "zero ppm." This works well on that scale because leftover runoff water is filtered and reused instead of drained off into the sewer system. (there's even rules about that run-off ... it in most cases needs to be treated before being released into a municipal wastewater system.)
I know I got a bit off topic ... but what about you ... the soon to be hydro grower. To better answer your questions and to be able to specifically tell you what to do will involve handing the hydro guru your water quality report. An expert can look at that report and tell you how to accomplish exactly what you want to do ... and whether or not RO is needed. In many cases, a simple carbon and sediment filtering system (2-stage) is all you need.