Lots of bad 411 offered here.
There are so many bad things in tap water that people should not shower, drink, cook, or use it as a nutrient base without first filtering- but that only covers the VOCs (don't be lazy, google it)
How anyone could possibly think that the composition of minerals (hardness) in municipal water is 'good' is equally absurd. AGAIN, google it
The more hardness, the greater the difficulty for nutes to mix properly. You can do a simple test with pure liquid soap adding one drop at a time then shaking. If you get a head of suds that do not dissolve quickly you have low hardness. Typically it takes 8-10 drops of soap in 10 ounces of water to overcome the hardness
Now, google DIY structured water. You can make a simple device using ~1-2" marbles in a ~ 10-24" tube. Pass tap through the top, then repeat the soap test. It's pretty amazing, only 1-3 drops needed.
I have a $2000 RO and still run that water through the structure tube