Tap water has a number different kinds of dissolved solids held insuspension in the water. When the water is purified these solids are removed.
Once the dirt is gone, the pure is all that’s left of the glass, and will dry clean and spot free.
If ordinary tap water was sprayed on to windows and left to dry it would result in the glass appearing milky white in colour when dried. These marks are caused by the mineral content of tap water. Pure Water is different because it is water without any mineral content so when it dries it leaves your windows spotlessly clean.
To ensure the water we are using is Pure we use a TDS Meter (Totally Dissolved Solids) to identify the quantity of minerals that are dissolved in tap water.