Why should you restring your racquet?
Most players only restrings their racquets only when the string has worn out or snapped, however it is best to restring your racquet once every 3 months. Strings are important as they can enhance your feel, comfort, and power. All strings degrade in quality almost immediately after your racquet is strung regardless of how often you play. This is because the strings are placed under constant stress and are adversely affected by, changes in temperature, exposure to sunlight and humidity.
To fully appreciate the benefits of fresh strings, you would need to consider what tension loss does to racquet performance.
More dwell time, less control…
When the string loses tension, it would cause the strings to stretch more resulting to a softer string bed, thus the dwell time of the ball /shuttlecock would be longer as the ball / shuttlecock stays on the strings longer. This will launch the ball / shuttlecock on a more unpredictable trajectory than you are used to. You can't figure out what technical flaw has emerged in your stroke, and you begin to mess with perfectly good mechanics to fix your mysterious condition.
More power, less control…
As tension goes down, the strings stretches more upon impact, returning almost all the energy that goes into stretching them (Newton’s 3rd Law), whatever the tension. Power is good if you want and can control it, and knowing how much to expect from day to day. That is how you grind your stroke--by responding the same way to the same situation. But your strings deliver varying amounts of power from day to day and from hit to hit. This works against grinding anything. As you struggle to keep the ball / shuttlecock in, you constantly change your stroke.