Motorcycle Tyres
It doesn't matter how good your bike is, if your tyres are worn or damaged you simply can't maintain maximum traction on the road, track or trail. It's not just tread depth that affects the condition of your tyres, though. Prolonged periods left stationary outside exposes the tyres to damage from the sun as well as the varying weather conditions, which can cause cracks and discolouration to appear.
The absolute minimum legal tyre tread depth is 1mm, but it is recommended to change the tyre once it gets to 2mm for your own safety, especially when you consider that the tyre's ability to displace water has now been massively reduced compared to its original depth. It is also generally recommended to keep a very watchful eye on tyres that are over 5 years old and replace them after 6 years to minimise the risk of internal structural damage that isn't visible from the outside.