So you have bought a tennis racket which you love - fantastic! That's only half the story though when it comes to choosing the right equipment to play with.
Many people forget about strings!
I always liken the racket and string relationship to an F1 car and its tyres. The car can perform very well, but when it has the wrong tyres on, the car will skid off the track. It is after all, the tyres which are touching the ground - or the strings which are actually making contact with the ball.
Why & When
Strings deteriorate over time meaning that you're not getting the most out of your racket. Through the course of a strings life it loses its elasticity, meaning the string does not fire back into position once the ball hits. This results in less movement, or rotation, on the ball. You may experience less control, less spin, your once perfect forehand starts sailing long and increased vibration (which does not help if you are prone to tennis elbow).
Restringing a racket isn't just for the string breakers out there!
If you care about your performance on court then you should be getting your racket restrung quite frequently.