Just because the strings have not broken, does not mean that they are still giving optimum performance. As soon as any item under tension is released creep will start to take effect. Hence the racquet strung today at 24lb, will settle to about 22lb by tomorrow. The higher the tension in the string bed, the less durable the string becomes.
Modern racquets with their wide body sections and stiffer frames, work the strings much harder than in a conventional racquet. This is a consequence of the racquets generating more power. Other factors affecting the string durability are players who generate a faster movement of the racquet head (hard hitters), those who hit a lot of slice shots and the use of shuttles with a hard base.
String movement is another common area of concern; strings do not need to be straight to offer maximum performance. The only strings that do not move are cheap, standard nylon (monofilament) strings. These are prevented from moving by notching into each other.
As a rule of thumb, you should be looking to restring your racquet the number of times a year, as you play in a week. (Even if it hasn't snapped). Many players prefer to have their racquets restrung even more so than that, as after a while, strings begin to pinch, wear or feel 'dead'.
In summary, you re-string because your strings will not stay elastic forever, and this eventually translates into loss of control and power.






