Wristbands are useful mainly where sweat is involved. If you don't play tennis with much intensity and don't sweat then wristbands act more like a fashion accessory. However if you do sweat while playing tennis or any racket sport for that matter then wristbands can be helpful in multiple ways. You can use them to wipe the sweat off your forehead which can be very useful as otherwise the sweat can drip down to your eyes distracting your vision. They also prevent the sweat from flowing through to your hands which is important as it makes the palms less sweaty and will help in gripping the racket better.