Firstly, because over a period of time, the frames gradually get cleaner, leaving no dirt for the rain to wash down onto the glass.
Secondly, because WFPS wash so efficiently with no detergents, they leave no detergent residue on the glass, which would otherwise attract particles of dirt. Eventually the glass becomes sterilized.
Of course you can use tap water together with a soap or detergent, but if you only use tap water to clean the windows, the glass will appear milky white and/or streaky when it dries, and you will be able to write on the glass surface with your finger. (Try it for yourself). This effect on the glass is due entirely to the mineral content of the tap water.
Only pure water allows for spot-free cleaning and so tap water must be processed through a reverse-osmosis unit (RO) or a de-ionisation vessel (DI) to remove all impurities before being used to clean windows.
TDS means Totally Dissolved Solids, and is the measurement (in parts per million) of the total quantity of minerals dissolved in water. (If you were to test the water at your tap, the meter would probably show a reading of between 035 parts per million (035 ppm) and 600 parts per million (600 ppm) depending where you are in the country).
RO means Reverse-Osmosis and refers to the reverse-osmosis filtration unit used to extract 95 to 98% of the dissolved solids that exist in all normal tap water.
DI means De-Ionisation, and is usually referred to in the context of “DI Cannister” or just “DI”. The “DI canister” is used to “polish” the water just before it is applied to the windows.
Pure Water means when all of the minerals are filtered or extracted from the water, resulting in a TDS reading of 000 ppm (indicating zero mineral content or total demineralisation)