1. obviously, the moisturing spray is for moisturing the skin, but some could even have the function for relieving the heat by temperature. I know that Clarins have a new spray for prevent the harmful molecules/radiation from computer and other pollutents.
2.just at the moment you need,but one thing is very important, you must apply lotions or cream after using moisturing spray in order to prevent further evaporation from skin.
3. depends on your personal needs, many people travel on the plane like using that, for easlier water replacement.
4. Try try shu uemura one, deep water moisturing spray cost $150