Water makes up 97% of beer (possibly more in some pubs ;o) ). The minerals and ions within that water have a dramatic effect on the characteristics of the beer.
Adjusting the concentrations and ratios of salts within your brewing water is a complex subject that many home brewers choose not to get into. However, if you do want to open that particular can of worms, it’s vital that you know the makeup of your supply.
This page contains data either measured by our members or obtained from their water suppliers to help you to baseline your water and design appropriate liquor treatment for your beers.