Clean windows are not just pleasing to the eye, but are an important part of maintaining a home. A little known fact about glass is that its surface is not entirely smooth, but quite porous and full of voids.
These voids can quickly accumulate dirt and grime, and over time obscure glass visibility. In combination with rain water the dirt and grime can form a film similar to the hard water deposits left from tap water.
The best way to avoid this buildup is by having your windows cleaned regularly. I recommend window cleaning be done twice annually.