The harder you hit, and the more spin you hit, the more likely the strings will break sooner. 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 touches them. String Savers are mostly used by players that have gut strings. They are usually effective at prolonging string life - but how much will vary.
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 or to string at a higher tension . 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.