Minimum legal tyre tread depth

The tread is there as stated for wet roads as each groove or sipe acts as a pump to remove the water from under it.

Slicks used on a race track are not just bald tyres, they are carefully engineered to heat up quickly without overheating or delaminating under severe conditions by using different compounds or materials.

As Rog already said, Trucks and Motorcycle tyres (1mm) have a lower legal tread depth than cars (1.6mm)