Correct fitting is important
Standard car tyres have a symmetrical tread pattern which is the same across the whole width of the tyre. The tread pattern on an asymmetric car tyre changes across the tread as different parts of that tread are adapted for particular functions.
The outside edge consists of large stiffer tread blocks which help with cornering. The inner tread blocks are smaller and designed to shift water and improve wet grip. In addition, the middle of the tread usually has a continuous rib to help straight line stability.
Asymmetrics must be fitted the right way round on the wheel to benefit from the tread block arrangement. The sidewalls are always marked.
Directional car tyres usually have a chevron or arrow pattern in the tread and are designed to work in a specific direction of rotation. Key benefits are:
Directional tyres need to rotate in the correct direction – clearly marked on the sidewall of the tyre.
If fitted the wrong way round, the tyre won't be dangerous, but you won't gain any of the benefits of a directional tyre.
Incorrect fitting of both asymmetric and directional tyres is a reason for NCT failure too. Some very high performance cars have directional and asymmetric tyres. These have to be fitted the right way round on the wheel and on the correct side of the vehicle.