The washer fluid, rain or snow on the windshield act as a lubricant for the wipers. In extremely DRY conditions there is a slight difference in wiper motor effort. The difference isn' t critical. Prolongs wiper blade life and makes it perform better, longer. Wiper blades are V-shaped.
Liquids act as lubricant.
All abrasive dirt is wipped off. Cleans off road film, bugs and dirt. In hot weather, the oils in road asphalt rise to the surface and are projected onto your windshield by other vehicles. Then, when it rains, the wipers spread this tacky film that smears your windshield. Bugs - The same result is obtained with squashed bugs as for road film.