The importance of clean windows doesn't need to be stated. However, sometimes environments cause the window to reduce visibility levels while the vehicle is being driven (due to rain, dust, humidity/fog, and other situations). The windshield wipers can only do so much, but there are ways to prepare a window for some of these environments, too.
The window's outside surface encounters the elements of the outside environment, such as rain, dust, and bugs. Most of what gets stuck to the window comes off in car washes, but it's good to have a spray on hand that can clean the window between washes. A little bit of car wash soap or bug remover spray will usually work, depending on the season.
Windshield wipers have to wipe water away because the water sticks to the dirt and fine pores in the glass. Treatments, such as the one by Rain-X to the right, are available for (temporarily) filling those microscopic pores so the water beads, allowing the wind passing over the window to blow away the water. Some treatments can be added to the windshield waster fluid, but aren't as effective as the rubbed-in treatment.
The inside surface of the window may not have to deal with the harshness of the outside environment, under the wrong circumstances, it can still make it difficult to see the road ahead. If the outside is cold and the inside is warm and humid, condensation may form (sometimes referred to as "fogging up the windows" due to its similar appearance to fog) on the inside. Skin oils (which stick to the window when you touch it) and other contaminants can make the condensation thicker and harder to get rid of.