Tyres should be checked on a regular basis to ensure that they are both safe and legal. If your tyre tread depth is below the UK legal limit of 1.6mm then you are not insured!
Worn tyres affect the performance of your vehicle and can reduce the effectiveness of your braking, steering and acceleration.
Take a 20p and place it in-between the main grooves of the tyre. If you can see the outer band of the coin, the tyre may have insufficient tread depth. At this point, it is advised that you have your tyres checked over.