We may have had the experience of encountering windshield washer fluid that gives out while driving in a storm or muddy conditions. It is important to have plenty of fluid on hand because it needs to last if a storm or inclement conditions continue for any length of time.
Ideally, the fluid should not be just water only. It should be a mixture of water detergent, and other materials, to sweep away the dirt and bugs. The debris on the windshield usually sweeps right off with the right mixture.
The washer fluid also acts as a lubricant for the wipers as they glide back and forth across the glass. If the surface of the glass does not get lubricated, the wiper blades have a tendency to "jump" which cause a strain on the motor that drives the wipers. Over time, this can cause the motor to malfunction, causing a costly repair to occur.
From time to time, check the level of the wiper fluid and make sure it is topped off, so you don't run out in a big storm.