In A Very Quick Introduction to Japanese Tea Brewing I make a claim:
Boiling the water removes a lot of the non-soluble minerals in the water, giving the tea a less harsh flavor.
@firstname.lastname@example.org on Fediverse sent a reply when I posted a link to that guide stating "Minerals don't evaporate, so I think you're more likely to concentrate them as a (small) portion of the water evaporates.", and I've spent some time unpacking all of this. My initial claim was non-soluble minerals would precipitate on boiling and that the remaining soluble minerals would allow for the tea to have more "places" to bind to. I'm not so sure it's that cut-and-dry any more, and in fact I think that the purpose of soluble minerals in tea may actually serve in mediating the tea extraction rather than encouraging it.
Now, this opens with the proviso that I am not a chemist, that I am a layperson, that I am a massive dumbass. We're all about Making Mistakes here and learning from them, so I may be as wrong as I was three hours ago, but this is my mental model:
Speaking in very broad strokes there are two "types" of minerals in water which we care about when Brewing Tea: Soluble and Non-soluble minerals. Consider common table salt, many salts can mix in to water and when they do there are not particles of salt suspended in it, but the water has electrically bonded to the molecules of the minerals. If you leave salt-water to sit, the salt does not "settle" to the bottom, but if you were to add, say iron hydroxide (aka rust) you would find that even with vigorous shaking, the solution would only be a temporary colloidal solution. Another common non-soluble mineral is Calcium Carbonate or Limestone – it's extremely common in some hard tap waters like what I find in Arizona. It's not uncommon for sink spouts to have a chalky white rim of lime scaling there.
Our bodies have evolved to give many of these minerals flavor, some of them are nice, and some of them are quite "sharp" or harsh. I haven't even begun to consider the evolutionary biological origins of the way our bodies interpret these flavors1, but luckily for us the soluble minerals in our water do not taste bitter or sharp or harsh, to many folks they taste like nothing. Likewise, the non-soluble minerals are minerals which we do not find to have a good taste, whether they have more acidity (lower pH) or are toxic or just don't confer any benefit to ingest like rust.
In both cases, boiling the solution will force some (not a lot!) of the water molecules to evaporate, but not the water molecules which are bound to soluble minerals because these take more energy to separate than the more readily boiling unbound molecules. So what you'll see happen is that trace amounts of the non-soluble minerals, mostly carbonates but some other things like calcium sulfate (gypsum) will settle as the water circulates and as the uhh "colloidal carrying capacity" of the water decreases and it will precipitate on the kettle creating limescale. You can see in the image above that my scale has a distinctly red tinge to it, indicating that there is likely an insoluble iron compound in my water supply. Meanwhile, soluble minerals will stay in the water, providing minute flavors while making the water a less effective solvent. And that last part is the key insight for me, which sort of flips my tea-brewing philosophy on its head.
In Brewing, we are extracting the soluble materials from the tea leaves when they rehydrate2. Mineral water will have less "soluble carrying capacity", let's call it, than pure distilled water, giving the tea more delicate or subtle tastes and letting the dominant polyphenol/astringent/caffeine/bitter flavors rest within the leaves. When the tea is brewed in harder water, the much more intense flavors within the tea are insoluble which lets the subtler flavors of the minerals and amino acids and that sort of stuff come forward. But if the hardness of the water confers new flavors itself, this may combat the subtle flavors of the tea.
A lot of the advice I've read online is that you should "never" brew with distilled or purified water – that the tea would taste "empty" or flat or without structure, and I assumed at the time that this had to do with the fact that the minerals would make the tea more soluble, that you're getting "more" tea, but the opposite is probably true. It may be the case that distilled water is much more ready to pull out the bitter parts of tea which a mineralized water may more readily leave behind and thus you have to be much more gentle, intentional, sensitive in brewing with distilled waters. But, that said, there are minerals with harsh flavors, it's maybe more about having the correct minerals in your water than none.
It's important to remember, and easy to forget, that most of the soluble "stuff" in tea is quite bitter, in fact, and it's only through mediating that that the more subtle and enjoyable parts of tea come forward. One way to mediate this is by brewing high-umami teas at lower temperatures because the amino acids are more readily soluble at lower temperatures than polyphenols and caffeine, but most teas aren't made to accentuate those flavors and are made to have boiling or fresh-boiled water, and the water's mineral content along with brew time may be a more effective mediator in these cases.