There are more reasons than most would think to keep your windows clean year round, the obvious reasons are: 
-A better view, fresh feel to your home
-heat efficiency
-professional business image at the office or home
Beyond the obvious reasons, the other important reason to clean your windows is to avoid glass deterioration. Window glass eventually becomes stained with a variety of contaminants:
-hard minerals (from sprinkler systems and building run-off)
-oxidization (from windows encased in metal frames and screens)
-acid rain
-over spray (from paint, chalking, mortar and more)
Hard minerals or hard water is the biggest threat 
The need for basic window glass restoration or replacement can be avoided with regular cleaning to exterior window surfaces.
It is recommend no less than two cleanings per year. To prevent permanent damage , and having to replace your windows eventually. Saving you money in the long run and a fresh look to your investment.
Understanding glass:
-glass is porous. a cross-section of a sheet of glass reveals peaks and valleys.
-glass is a liquid that continues to harden over time.
Since glass is porous, it’s easy to see how it’s transparent look can quickly become obstructed with airborne dirt and pollution. 
As well , why hard water (rain water) runs over  painted siding, roof shingles etc…and leaves minerals and acids in the pores of the glass.    
               Exterior-Home-Designs-662                      10300810_647916921965595_1655255237988220802_n