They prevent your strings from rubbing against each other, and are intended to help prolong string life. String Savers are legal because the ball never actually touchs them. String Savers are mostly used by players that have gut strings.
Some players complain that they fall out, particularly if they hit with a lot of spin.
Some players also do not like String Savers because the give the racquet a stiffer feel. Another alternative to lengthen string life is to use a thicker guage string. In any event, it is still recommended to replace your strings at least as many times in a year as you play on average in a week. There is a little picture on the String Savers box showing how to install them, along with a tool for insertion. One box includes enough for one racquet.