When growing extreme quality cannabis, or duplicating hydro results, R/O levels the playing field.
Yes, there are some water supplies which are as clean as R/O. They are the exception and not the rule. Some tap water contains only dissolved elements cannabis can use as food. Again, this is the exception and not the rule.
The unfortunate part about cannabis is its a hyper/dynamic accumulator flower and not a fruit or vegetable. This means any non-food or excessive elements available will decrease quality in ways which cannot be flushed/faded out. The absorption of elements, through other channels than passive ionic uptake, and its ability to directly bind elements to new growth unconverted, makes a huge difference where quality is concerned.
A lot of people have difficulty telling the difference between 2% and whole milk, or butter and margarine. For these growers/tokers, 'decent' cannabis is the bomb. Those who have gained awareness, of what truly clean cannabis is like, are those who prefer R/O grown and faded cannabis. Those who prefer exactly predictable pH changes, or are looking for duplicateable results (which includes newbies), have the easiest learning curve with R/O and advice from other R/O growers.
The other nice thing is not having to re-learn or adjust your methods when you move to a new water supply. Lots of examples of folks who've had that issue.
This is why I always recommend R/O water, regardless of grow method.