How would you like to be able to clean your windows without having to take out each window screen? That is the advantage of vanishing screens on your windows. Most of the time you don't have your windows open, so why would you ALWAYS want to be looking through a dirty screen? You don't.
Vanishing screens are mounted to most any kind of operable windows. The most popular seems to be the kitchen window as a pass through to the back patio barbeque area.