Both Natrium and Chlorine attract a comparatively big amounts of hydrates around them so their molecules block a lot of entrance space at surface of the roots, where the roots exchange H+ and other ions or chelate-builder substances (acids, malat etc).
The optimal case would be if the plant only attracts ions which are essential or beneficial to a plant. Plants have multiple ways to deal with unwanted attracted ions so they fill chambers inside or outside to store er excreet these salts.
They likewise do so with excess Ca or other minerals. Na for Cannabis is a complete waste of potential, it's not needed and is also not beneficial, at least, that I know off. There's plants that like Na but these are oceanic plants. Conversely, Cl is essential but in such a tiny amount you can skip it, too. In soil it's abundant anyway. It's less needed than any micro, by far. So basically everything you feed a plant of it will be treated as excess.
Plants can drive up metabolic responses to deal with these unwanted substances but at the cost of energy. And sometimes these molecules are used accidentally and integrated into generic plant tissue.
Rainwater has zero electrical conductivity and only very minicscule acidic pH, when it hits the ground and trickles into earth it will solve some of the humic & fulvic acids whicha re found in the top humic sphere of the soil and then later on, as it reaches depper and deeper, gets enriched by alot of other substances as well. Basically alot of what the water surpasses, ie. deep down earth is rather rocky - and thats where then this enormously enrichment with minerals like chalk & magnesia stems from. That's why tap water is so hard - in comparison to what plants usually get to drink in nature. Tap water covered alot of rocky soil deep down - miles, it travels largely horizontal... so deep down usually no roots develop.
We can grow a plant by enriching RO with a full fertilizer to EC 1.2 and inside that range is everything contained that the plant needs to thrive. Most of what makes this out are the macroelements NPK Ca S Mg and the micros. If we take tapwater inside which is, to make an example, hard in EC 0.7 pH 8.5 (alkaline) we would increase the saltiness of the feed solution considerably to EC 1.9. A plant can tolerate this, but EC 1.2 isn't even high.... but if we go further than EC 3.1 the plant is going to die. Saltiness itself has an effect on the osmotic pressure which the plant experiences as it takes the water in.
Because of this, it's sort of more "smooth" to use plain water esp. rainwater.
Another thing is amount of generally unhealthy substances such as PAC or nitrit etc pp - if you feed those, they'll all get combusted, too. So this is why I actually see my RO water generator as a one step for me to do a filter and clearing of a very important source of my household - I wish my showering water could be filtered.