Re. #2 - Les and David are both right. They impede insects a little, but mostly they cut down on whistling noise from passing air. Many, including myself, have found a noticeable change in wind noise when installing them over the open weep holes of the driver's and copilot's side windows where the covers had been missing unnoticed for awhile.
The weep hole covers do come off easily, especially as they age. Replacements unfortunately come in different sizes and commonly are designated for the right or left side of the coach so any streamlined end is facing the front. They can be found online and often at RV dealerships, but be sure you're getting the right ones for your specific coach/window mfr., and order several of each, left and right, for future replacements.
Joel