Beer is mostly water, and the dissolved minerals in that water change how a brew ferments and tastes. Before water could be treated, each brewing city was limited to the styles its supply happened to suit.
Minerals change the mash before they change the taste
Enzymes that convert starch to sugar during mashing work best within a narrow acidity range, and dissolved carbonate raises acidity in the wrong direction.
Dark malts are acidic, so they pull the mash back down. A city with hard, carbonate-rich water could brew dark beer successfully and struggled with pale.
Soft water with little carbonate suits pale malts, which do not supply the same corrective acidity.
Sulphate and chloride pull in opposite directions
Sulphate accentuates hop bitterness and gives it a dry, crisp finish.
Chloride does the reverse, rounding the palate and emphasising malt sweetness and body.
The ratio between the two is the single most-discussed number in brewing water, because it moves a beer along the axis from bitter and dry to soft and full.
Famous styles map onto famous water
Regions with very high sulphate produced assertively bitter pale ales, and the mineral character became part of what the style meant.
Regions with very soft water produced pale lagers with delicate bitterness that would taste harsh if brewed with harder water.
Cities with carbonate-heavy supplies built their reputations on dark styles, because those were the beers that worked reliably.
Calcium does the structural work
Calcium stabilises enzymes during the mash, helps proteins settle out after the boil, and encourages yeast to flocculate and drop clear once fermentation finishes.
Water very low in calcium ferments sluggishly and clears poorly, which is why calcium salts are the first addition most brewers make.
Its own taste contribution is minimal, so it is adjusted for process reasons rather than for flavour.
Treatment removed the constraint
Brewers can now strip minerals by reverse osmosis and rebuild the profile with measured salt additions.
Any style can therefore be brewed anywhere, and water profiles from historic brewing cities are reproduced deliberately as recipes.
The constraint that created the styles disappeared, but the styles it created outlived it.
Why brewers still publish their water profile
Water treatment is a design decision, and small changes shift perceived bitterness noticeably at the same measured hop level.
Hazy hop-forward styles are typically brewed with high chloride and low sulphate to produce a soft mouthfeel, which is as much a part of the style as the hops are.
Publishing the figures also lets other brewers reproduce a result, which matters in a field where recipes are shared far more openly than in most drinks industries.