Pros carry many rackets because a string or a frame could break over the course of a match and if you don't have any backup rackets, you certainly wouldn't be able to play if such a thing happened. Also, during the course of playing with a racket, the string tension decreases, because the strings are gradually getting marginally looser. Decreased string tension can lead to decreased control. You will often see a professional player pull out a freshly strung racket every 7 games when they traditionally switch to newer balls. This is all a measure to improve their accuracy. Also, many players will play with rackets strung at slightly different tensions (I'm certainly no pro, but I like having 2 rackets strung at 57 pounds and 2 at 58). This way, they can adjust and tweak to produce better results.