They are for emergency use, you can say which mile marker you are closest to on 911 calls so they know where to find you if you aren't near an exit. Also, exits are usually numbered by the mile markers, so it's assuring you that you didn't (or did) miss your exit. Many states have them, though they are usually small and posted on the guard rails. I've seen ones that are very thin and use decimals in a vertical fashion (105.2, with each number and the dot is a horizontal line).
