There are cartridges with minerals in the reverse osmosis filter system that release the appropriate minerals to the water since only water can pass from the first stage. That is because the human body needs them. If the water has no minerals, like the water collected from rain, the water used in irons for the clothes, then this water will absorb minerals from the body and the body will lose minerals it needs and after some time, not that soon, there will be a problem, not sure what. I am not an expert on this, but that is what an expert has told me.
Also i like reading history books sometimes. In the past during sieges of cities, the sieged people had discovered that the rain water was not that good, so they threw some lime (i think) in the water containers, not sure what exactly the thinking was though back then.
The conclusion is anyway, that water has to have a certain amount of minerals to be good. No more, no less.
I am sure you will "investigate" the subject and you will find out. The expert had told me these since years. Maybe what i said is outdated 🤷